diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
index 42942e43..727a97d9 100644
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -1,4053 +1,4052 @@
 <?php
 
 /**
  * Calendar plugin for Roundcube webmail
  *
  * @author Lazlo Westerhof <hello@lazlo.me>
  * @author Thomas Bruederli <bruederli@kolabsys.com>
  *
  * Copyright (C) 2010, Lazlo Westerhof <hello@lazlo.me>
  * Copyright (C) 2014-2015, Kolab Systems AG <contact@kolabsys.com>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
  * published by the Free Software Foundation, either version 3 of the
  * License, or (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU Affero General Public License for more details.
  *
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
 #[AllowDynamicProperties]
 class calendar extends rcube_plugin
 {
     const FREEBUSY_UNKNOWN   = 0;
     const FREEBUSY_FREE      = 1;
     const FREEBUSY_BUSY      = 2;
     const FREEBUSY_TENTATIVE = 3;
     const FREEBUSY_OOF       = 4;
 
     const SESSION_KEY = 'calendar_temp';
 
     public $task = '?(?!logout).*';
     public $rc;
     public $lib;
     public $resources_dir;
     public $home;  // declare public to be used in other classes
     public $urlbase;
     public $timezone;
     public $timezone_offset;
     public $gmt_offset;
     public $dst_active;
     public $ui;
 
     public $defaults = [
         'calendar_default_view' => "agendaWeek",
         'calendar_timeslots'    => 2,
         'calendar_work_start'   => 6,
         'calendar_work_end'     => 18,
         'calendar_agenda_range' => 60,
         'calendar_show_weekno'  => 0,
         'calendar_first_day'    => 1,
         'calendar_first_hour'   => 6,
         'calendar_time_format'  => null,
         'calendar_event_coloring'      => 0,
         'calendar_time_indicator'      => true,
         'calendar_allow_invite_shared' => false,
         'calendar_itip_send_option'    => 3,
         'calendar_itip_after_action'   => 0,
     ];
 
     // These are implemented with __get()
     //  private $ical;
     //  private $itip;
     //  private $driver;
 
 
     /**
      * Plugin initialization.
      */
     function init()
     {
         $this->rc = rcube::get_instance();
 
         $this->register_task('calendar', 'calendar');
 
         // load calendar configuration
         $this->load_config();
 
         // catch iTIP confirmation requests that don're require a valid session
         if ($this->rc->action == 'attend' && !empty($_REQUEST['_t'])) {
             $this->add_hook('startup', [$this, 'itip_attend_response']);
         }
         else if ($this->rc->action == 'feed' && !empty($_REQUEST['_cal'])) {
             $this->add_hook('startup', [$this, 'ical_feed_export']);
         }
         else if ($this->rc->task != 'login') {
             // default startup routine
             $this->add_hook('startup', [$this, 'startup']);
         }
 
         $this->add_hook('user_delete', [$this, 'user_delete']);
     }
 
     /**
      * Setup basic plugin environment and UI
      */
     protected function setup()
     {
         $this->require_plugin('libcalendaring');
         $this->require_plugin('libkolab');
 
         $this->lib             = libcalendaring::get_instance();
         $this->timezone        = $this->lib->timezone;
         $this->gmt_offset      = $this->lib->gmt_offset;
         $this->dst_active      = $this->lib->dst_active;
         $this->timezone_offset = $this->gmt_offset / 3600 - $this->dst_active;
 
         // load localizations
         $this->add_texts('localization/', $this->rc->task == 'calendar' && (!$this->rc->action || $this->rc->action == 'print'));
 
         require($this->home . '/lib/calendar_ui.php');
         $this->ui = new calendar_ui($this);
     }
 
     /**
      * Startup hook
      */
     public function startup($args)
     {
         // the calendar module can be enabled/disabled by the kolab_auth plugin
         if ($this->rc->config->get('calendar_disabled', false)
             || !$this->rc->config->get('calendar_enabled', true)
         ) {
             return;
         }
 
         $this->setup();
 
         // load Calendar user interface
         if (!$this->rc->output->ajax_call
             && (empty($this->rc->output->env['framed']) || $args['action'] == 'preview')
         ) {
             $this->ui->init();
 
             // settings are required in (almost) every GUI step
             if ($args['action'] != 'attend') {
                 $this->rc->output->set_env('calendar_settings', $this->load_settings());
             }
         }
 
         if ($args['task'] == 'calendar' && $args['action'] != 'save-pref') {
             if ($args['action'] != 'upload') {
                 $this->load_driver();
             }
 
             // register calendar actions
             $this->register_action('index', [$this, 'calendar_view']);
             $this->register_action('event', [$this, 'event_action']);
             $this->register_action('calendar', [$this, 'calendar_action']);
             $this->register_action('count', [$this, 'count_events']);
             $this->register_action('load_events', [$this, 'load_events']);
             $this->register_action('export_events', [$this, 'export_events']);
             $this->register_action('import_events', [$this, 'import_events']);
             $this->register_action('upload', [$this, 'attachment_upload']);
             $this->register_action('get-attachment', [$this, 'attachment_get']);
             $this->register_action('freebusy-status', [$this, 'freebusy_status']);
             $this->register_action('freebusy-times', [$this, 'freebusy_times']);
             $this->register_action('randomdata', [$this, 'generate_randomdata']);
             $this->register_action('print', [$this,'print_view']);
             $this->register_action('mailimportitip', [$this, 'mail_import_itip']);
             $this->register_action('mailimportattach', [$this, 'mail_import_attachment']);
             $this->register_action('dialog-ui', [$this, 'mail_message2event']);
             $this->register_action('check-recent', [$this, 'check_recent']);
             $this->register_action('itip-status', [$this, 'event_itip_status']);
             $this->register_action('itip-remove', [$this, 'event_itip_remove']);
             $this->register_action('itip-decline-reply', [$this, 'mail_itip_decline_reply']);
             $this->register_action('itip-delegate', [$this, 'mail_itip_delegate']);
             $this->register_action('resources-list', [$this, 'resources_list']);
             $this->register_action('resources-owner', [$this, 'resources_owner']);
             $this->register_action('resources-calendar', [$this, 'resources_calendar']);
             $this->register_action('resources-autocomplete', [$this, 'resources_autocomplete']);
             $this->register_action('talk-room-create', [$this, 'talk_room_create']);
 
             $this->add_hook('refresh', [$this, 'refresh']);
 
             // remove undo information...
             if (!empty($_SESSION['calendar_event_undo'])) {
                 $undo = $_SESSION['calendar_event_undo'];
                 // ...after timeout
                 $undo_time = $this->rc->config->get('undo_timeout', 0);
                 if ($undo['ts'] < time() - $undo_time) {
                     $this->rc->session->remove('calendar_event_undo');
                     // @TODO: do EXPUNGE on kolab objects?
                 }
             }
         }
         else if ($args['task'] == 'settings') {
             // add hooks for Calendar settings
             $this->add_hook('preferences_sections_list', [$this, 'preferences_sections_list']);
             $this->add_hook('preferences_list', [$this, 'preferences_list']);
             $this->add_hook('preferences_save', [$this, 'preferences_save']);
         }
         else if ($args['task'] == 'mail') {
             // hooks to catch event invitations on incoming mails
             if ($args['action'] == 'show' || $args['action'] == 'preview') {
                 $this->add_hook('template_object_messagebody', [$this, 'mail_messagebody_html']);
             }
 
             // add 'Create event' item to message menu
             if ($this->api->output->type == 'html' && (empty($_GET['_rel']) || $_GET['_rel'] != 'event')) {
                 $this->api->output->add_label('calendar.createfrommail');
                 $this->api->add_content(
                     html::tag('li', ['role' => 'menuitem'],
                         $this->api->output->button([
                             'command'  => 'calendar-create-from-mail',
                             'label'    => 'calendar.createfrommail',
                             'type'     => 'link',
                             'classact' => 'icon calendarlink active',
                             'class'    => 'icon calendarlink disabled',
                             'innerclass' => 'icon calendar',
                         ])
                     ),
                     'messagemenu'
                 );
             }
 
             $this->add_hook('messages_list', [$this, 'mail_messages_list']);
             $this->add_hook('message_compose', [$this, 'mail_message_compose']);
         }
         else if ($args['task'] == 'addressbook') {
             if ($this->rc->config->get('calendar_contact_birthdays')) {
                 $this->add_hook('contact_update', [$this, 'contact_update']);
                 $this->add_hook('contact_create', [$this, 'contact_update']);
             }
         }
 
         // add hooks to display alarms
         $this->add_hook('pending_alarms', [$this, 'pending_alarms']);
         $this->add_hook('dismiss_alarms', [$this, 'dismiss_alarms']);
     }
 
     /**
      * Helper method to load the backend driver according to local config
      */
     private function load_driver()
     {
         if (!empty($this->driver)) {
             return;
         }
 
         $driver_name = $this->rc->config->get('calendar_driver', 'database');
         $driver_class = $driver_name . '_driver';
 
         require_once($this->home . '/drivers/calendar_driver.php');
         require_once($this->home . '/drivers/' . $driver_name . '/' . $driver_class . '.php');
 
         $this->driver = new $driver_class($this);
 
         if ($this->driver->undelete) {
             $this->driver->undelete = $this->rc->config->get('undo_timeout', 0) > 0;
         }
     }
 
     /**
      * Load iTIP functions
      */
     private function load_itip()
     {
         if (empty($this->itip)) {
             require_once($this->home . '/lib/calendar_itip.php');
             $this->itip = new calendar_itip($this);
 
             if ($this->rc->config->get('kolab_invitation_calendars')) {
                 $this->itip->set_rsvp_actions(['accepted','tentative','declined','delegated','needs-action']);
             }
         }
 
         return $this->itip;
     }
 
     /**
      * Load iCalendar functions
      */
     public function get_ical()
     {
         if (empty($this->ical)) {
             $this->ical = libcalendaring::get_ical();
         }
 
         return $this->ical;
     }
 
     /**
      * Get properties of the calendar this user has specified as default
      */
     public function get_default_calendar($calendars = null)
     {
         if ($calendars === null) {
             $filter    = calendar_driver::FILTER_PERSONAL | calendar_driver::FILTER_WRITEABLE;
             $calendars = $this->driver->list_calendars($filter);
         }
 
         $default_id = $this->rc->config->get('calendar_default_calendar');
         $calendar   = !empty($calendars[$default_id]) ? $calendars[$default_id] : null;
         $first      = null;
 
         if (!$calendar) {
             foreach ($calendars as $cal) {
                 if (!empty($cal['default']) && $cal['editable']) {
                     $calendar = $cal;
                 }
                 if ($cal['editable']) {
                     $first = $cal;
                 }
             }
         }
 
         return $calendar ?: $first;
     }
 
     /**
      * Render the main calendar view from skin template
      */
     function calendar_view()
     {
         $this->rc->output->set_pagetitle($this->gettext('calendar'));
 
         // Add JS files to the page header
         $this->ui->addJS();
 
         $this->ui->init_templates();
         $this->rc->output->add_label('lowest','low','normal','high','highest','delete',
             'cancel','uploading','noemailwarning','close'
         );
 
         // initialize attendees autocompletion
         $this->rc->autocomplete_init();
 
         $this->rc->output->set_env('timezone', $this->timezone->getName());
         $this->rc->output->set_env('calendar_driver', $this->rc->config->get('calendar_driver'), false);
         $this->rc->output->set_env('calendar_resources', (bool)$this->rc->config->get('calendar_resources_driver'));
         $this->rc->output->set_env('identities-selector', $this->ui->identity_select([
                 'id'         => 'edit-identities-list',
                 'aria-label' => $this->gettext('roleorganizer'),
                 'class'      => 'form-control custom-select',
         ]));
 
         $view = rcube_utils::get_input_value('view', rcube_utils::INPUT_GPC);
         if (in_array($view, ['agendaWeek', 'agendaDay', 'month', 'list'])) {
             $this->rc->output->set_env('view', $view);
         }
 
         if ($date = rcube_utils::get_input_value('date', rcube_utils::INPUT_GPC)) {
             $this->rc->output->set_env('date', $date);
         }
 
         if ($msgref = rcube_utils::get_input_value('itip', rcube_utils::INPUT_GPC)) {
             $this->rc->output->set_env('itip_events', $this->itip_events($msgref));
         }
 
         $this->rc->output->send('calendar.calendar');
     }
 
     /**
      * Handler for preferences_sections_list hook.
      * Adds Calendar settings sections into preferences sections list.
      *
      * @param array Original parameters
      *
      * @return array Modified parameters
      */
     function preferences_sections_list($p)
     {
         $p['list']['calendar'] = [
             'id'      => 'calendar',
             'section' => $this->gettext('calendar'),
         ];
 
         return $p;
     }
 
     /**
      * Handler for preferences_list hook.
      * Adds options blocks into Calendar settings sections in Preferences.
      *
      * @param array Original parameters
      *
      * @return array Modified parameters
      */
     function preferences_list($p)
     {
         if ($p['section'] != 'calendar') {
             return $p;
         }
 
         $no_override = array_flip((array) $this->rc->config->get('dont_override'));
 
         $p['blocks']['view']['name'] = $this->gettext('mainoptions');
 
         if (!isset($no_override['calendar_default_view'])) {
             if (empty($p['current'])) {
                 $p['blocks']['view']['content'] = true;
                 return $p;
             }
 
             $field_id = 'rcmfd_default_view';
             $view = $this->rc->config->get('calendar_default_view', $this->defaults['calendar_default_view']);
 
             $select = new html_select(['name' => '_default_view', 'id' => $field_id]);
             $select->add($this->gettext('day'), "agendaDay");
             $select->add($this->gettext('week'), "agendaWeek");
             $select->add($this->gettext('month'), "month");
             $select->add($this->gettext('agenda'), "list");
 
             $p['blocks']['view']['options']['default_view'] = [
                 'title'   => html::label($field_id, rcube::Q($this->gettext('default_view'))),
                 'content' => $select->show($view == 'table' ? 'list' : $view),
             ];
         }
 
         if (!isset($no_override['calendar_timeslots'])) {
             if (empty($p['current'])) {
                 $p['blocks']['view']['content'] = true;
                 return $p;
             }
 
             $field_id  = 'rcmfd_timeslots';
             $choices   = ['1', '2', '3', '4', '6'];
             $timeslots = $this->rc->config->get('calendar_timeslots', $this->defaults['calendar_timeslots']);
 
             $select = new html_select(['name' => '_timeslots', 'id' => $field_id]);
             $select->add($choices, $choices);
 
             $p['blocks']['view']['options']['timeslots'] = [
                 'title' => html::label($field_id, rcube::Q($this->gettext('timeslots'))),
                 'content' => $select->show(strval($timeslots)),
             ];
         }
 
         if (!isset($no_override['calendar_first_day'])) {
             if (empty($p['current'])) {
                 $p['blocks']['view']['content'] = true;
                 return $p;
             }
 
             $field_id  = 'rcmfd_firstday';
             $first_day = $this->rc->config->get('calendar_first_day', $this->defaults['calendar_first_day']);
 
             $select = new html_select(['name' => '_first_day', 'id' => $field_id]);
             $select->add($this->gettext('sunday'), '0');
             $select->add($this->gettext('monday'), '1');
             $select->add($this->gettext('tuesday'), '2');
             $select->add($this->gettext('wednesday'), '3');
             $select->add($this->gettext('thursday'), '4');
             $select->add($this->gettext('friday'), '5');
             $select->add($this->gettext('saturday'), '6');
 
             $p['blocks']['view']['options']['first_day'] = [
                 'title'   => html::label($field_id, rcube::Q($this->gettext('first_day'))),
                 'content' => $select->show(strval($first_day)),
             ];
         }
 
         if (!isset($no_override['calendar_first_hour'])) {
             if (empty($p['current'])) {
                 $p['blocks']['view']['content'] = true;
                 return $p;
             }
 
             $first_hour  = $this->rc->config->get('calendar_first_hour', $this->defaults['calendar_first_hour']);
             $time_format = $this->rc->config->get('calendar_time_format', $this->defaults['calendar_time_format']);
             $time_format = $this->rc->config->get('time_format', libcalendaring::to_php_date_format($time_format));
             $field_id    = 'rcmfd_firsthour';
 
             $select_hours = new html_select(['name' => '_first_hour', 'id' => $field_id]);
             for ($h = 0; $h < 24; $h++) {
                 $select_hours->add(date($time_format, mktime($h, 0, 0)), $h);
             }
 
             $p['blocks']['view']['options']['first_hour'] = [
                 'title'   => html::label($field_id, rcube::Q($this->gettext('first_hour'))),
                 'content' => $select_hours->show($first_hour),
             ];
         }
 
         if (!isset($no_override['calendar_work_start'])) {
             if (empty($p['current'])) {
                 $p['blocks']['view']['content'] = true;
                 return $p;
             }
 
             $field_id   = 'rcmfd_workstart';
             $work_start = $this->rc->config->get('calendar_work_start', $this->defaults['calendar_work_start']);
             $work_end   = $this->rc->config->get('calendar_work_end', $this->defaults['calendar_work_end']);
 
             $p['blocks']['view']['options']['workinghours'] = [
                 'title'   => html::label($field_id, rcube::Q($this->gettext('workinghours'))),
                 'content' => html::div('input-group',
                     $select_hours->show($work_start, ['name' => '_work_start', 'id' => $field_id])
                     . html::span('input-group-append input-group-prepend', html::span('input-group-text',' &mdash; '))
                     . $select_hours->show($work_end, ['name' => '_work_end', 'id' => $field_id])
                 )
             ];
         }
 
         if (!isset($no_override['calendar_event_coloring'])) {
             if (empty($p['current'])) {
                 $p['blocks']['view']['content'] = true;
                 return $p;
             }
 
             $field_id = 'rcmfd_coloring';
             $mode     = $this->rc->config->get('calendar_event_coloring', $this->defaults['calendar_event_coloring']);
 
             $select_colors = new html_select(['name' => '_event_coloring', 'id' => $field_id]);
             $select_colors->add($this->gettext('coloringmode0'), 0);
             $select_colors->add($this->gettext('coloringmode1'), 1);
             $select_colors->add($this->gettext('coloringmode2'), 2);
             $select_colors->add($this->gettext('coloringmode3'), 3);
 
             $p['blocks']['view']['options']['eventcolors'] = [
                 'title'   => html::label($field_id, rcube::Q($this->gettext('eventcoloring'))),
                 'content' => $select_colors->show($mode),
             ];
         }
 
         // loading driver is expensive, don't do it if not needed
         $this->load_driver();
 
         if (!isset($no_override['calendar_default_alarm_type']) || !isset($no_override['calendar_default_alarm_offset'])) {
             if (empty($p['current'])) {
                 $p['blocks']['view']['content'] = true;
                 return $p;
             }
 
             $alarm_type = $alarm_offset = '';
 
             if (!isset($no_override['calendar_default_alarm_type'])) {
                 $field_id    = 'rcmfd_alarm';
                 $select_type = new html_select(['name' => '_alarm_type', 'id' => $field_id]);
                 $select_type->add($this->gettext('none'), '');
 
                 foreach ($this->driver->alarm_types as $type) {
                     $select_type->add($this->rc->gettext(strtolower("alarm{$type}option"), 'libcalendaring'), $type);
                 }
 
                 $alarm_type = $select_type->show($this->rc->config->get('calendar_default_alarm_type', ''));
             }
 
             if (!isset($no_override['calendar_default_alarm_offset'])) {
                 $field_id      = 'rcmfd_alarm';
                 $input_value   = new html_inputfield(['name' => '_alarm_value', 'id' => $field_id . 'value', 'size' => 3]);
                 $select_offset = new html_select(['name' => '_alarm_offset', 'id' => $field_id . 'offset']);
 
                 foreach (['-M','-H','-D','+M','+H','+D'] as $trigger) {
                     $select_offset->add($this->rc->gettext('trigger' . $trigger, 'libcalendaring'), $trigger);
                 }
 
                 $preset = libcalendaring::parse_alarm_value($this->rc->config->get('calendar_default_alarm_offset', '-15M'));
                 $alarm_offset = $input_value->show($preset[0]) . ' ' . $select_offset->show($preset[1]);
             }
 
             $p['blocks']['view']['options']['alarmtype'] = [
                 'title'   => html::label($field_id, rcube::Q($this->gettext('defaultalarmtype'))),
                 'content' => html::div('input-group', $alarm_type . ' ' . $alarm_offset),
             ];
         }
 
         if (!isset($no_override['calendar_default_calendar'])) {
             if (empty($p['current'])) {
                 $p['blocks']['view']['content'] = true;
                 return $p;
             }
 
             // default calendar selection
             $field_id   = 'rcmfd_default_calendar';
             $filter     = calendar_driver::FILTER_PERSONAL | calendar_driver::FILTER_ACTIVE | calendar_driver::FILTER_INSERTABLE;
             $select_cal = new html_select(['name' => '_default_calendar', 'id' => $field_id, 'is_escaped' => true]);
 
             $default_calendar = null;
             foreach ((array) $this->driver->list_calendars($filter) as $id => $prop) {
                 $select_cal->add($prop['name'], strval($id));
                 if (!empty($prop['default'])) {
                     $default_calendar = $id;
                 }
             }
 
             $p['blocks']['view']['options']['defaultcalendar'] = [
                 'title'   => html::label($field_id, rcube::Q($this->gettext('defaultcalendar'))),
                 'content' => $select_cal->show($this->rc->config->get('calendar_default_calendar', $default_calendar)),
             ];
         }
 
         if (!isset($no_override['calendar_show_weekno'])) {
             if (empty($p['current'])) {
                 $p['blocks']['view']['content'] = true;
                 return $p;
             }
 
             $field_id   = 'rcmfd_show_weekno';
             $select = new html_select(['name' => '_show_weekno', 'id' => $field_id]);
             $select->add($this->gettext('weeknonone'), -1);
             $select->add($this->gettext('weeknodatepicker'), 0);
             $select->add($this->gettext('weeknoall'), 1);
 
             $p['blocks']['view']['options']['show_weekno'] = [
                 'title'   => html::label($field_id, rcube::Q($this->gettext('showweekno'))),
                 'content' => $select->show(intval($this->rc->config->get('calendar_show_weekno'))),
             ];
         }
 
         $p['blocks']['itip']['name'] = $this->gettext('itipoptions');
 
         // Invitations handling
         if (!isset($no_override['calendar_itip_after_action'])) {
             if (empty($p['current'])) {
                 $p['blocks']['itip']['content'] = true;
                 return $p;
             }
 
             $field_id = 'rcmfd_after_action';
             $select   = new html_select([
                     'name'     => '_after_action',
                     'id'       => $field_id,
                     'onchange' => "\$('#{$field_id}_select')[this.value == 4 ? 'show' : 'hide']()"
             ]);
 
             $select->add($this->gettext('afternothing'), '');
             $select->add($this->gettext('aftertrash'), 1);
             $select->add($this->gettext('afterdelete'), 2);
             $select->add($this->gettext('afterflagdeleted'), 3);
             $select->add($this->gettext('aftermoveto'), 4);
 
             $val    = $this->rc->config->get('calendar_itip_after_action', $this->defaults['calendar_itip_after_action']);
             $folder = null;
 
             if ($val !== null && $val !== '' && !is_int($val)) {
                 $folder = $val;
                 $val    = 4;
             }
 
             $folders = $this->rc->folder_selector([
                     'id'            => $field_id . '_select',
                     'name'          => '_after_action_folder',
                     'maxlength'     => 30,
                     'folder_filter' => 'mail',
                     'folder_rights' => 'w',
                     'style'         => $val !== 4 ? 'display:none' : '',
             ]);
 
             $p['blocks']['itip']['options']['after_action'] = [
                 'title'   => html::label($field_id, rcube::Q($this->gettext('afteraction'))),
                 'content' => html::div(
                     'input-group input-group-combo',
                     $select->show($val) . $folders->show($folder)
                 ),
             ];
         }
 
         // category definitions
         if (empty($this->driver->nocategories) && !isset($no_override['calendar_categories'])) {
             $p['blocks']['categories']['name'] = $this->gettext('categories');
 
             if (empty($p['current'])) {
                 $p['blocks']['categories']['content'] = true;
                 return $p;
             }
 
             $categories      = (array) $this->driver->list_categories();
             $categories_list = '';
 
             foreach ($categories as $name => $color) {
                 $key = md5($name);
                 $field_class = 'rcmfd_category_' . str_replace(' ', '_', $name);
                 $category_remove = html::span('input-group-append',
                     html::a([
                             'class'   => 'button icon delete input-group-text',
                             'onclick' => '$(this).parent().parent().remove()',
                             'title'   => $this->gettext('remove_category'),
                             'href'    => '#rcmfd_new_category',
                         ],
                         html::span('inner', $this->gettext('delete'))
                     )
                 );
 
                 $category_name  = new html_inputfield(array('name' => "_categories[$key]", 'class' => $field_class, 'size' => 30, 'disabled' => $this->driver->categoriesimmutable));
                 $category_color = new html_inputfield(array('name' => "_colors[$key]", 'class' => "$field_class colors", 'size' => 6));
                 $hidden         = '';
 
                 if (!empty($this->driver->categoriesimmutable)) {
                     $hidden =  html::tag('input', ['type' => 'hidden', 'name' => "_categories[$key]", 'value' => $name]);
                 }
 
                 $categories_list .= $hidden
                     . html::div('input-group', $category_name->show($name) . $category_color->show($color) . $category_remove);
             }
 
             $p['blocks']['categories']['options']['category_' . $name] = [
                 'content' => html::div(['id' => 'calendarcategories'], $categories_list),
             ];
 
             $field_id = 'rcmfd_new_category';
             $new_category = new html_inputfield(['name' => '_new_category', 'id' => $field_id, 'size' => 30]);
             $add_category = html::span('input-group-append',
                 html::a(
                     [
                         'type'    => 'button',
                         'class'   => 'button create input-group-text',
                         'title'   => $this->gettext('add_category'),
                         'onclick' => 'rcube_calendar_add_category()',
                         'href'    => '#rcmfd_new_category',
                     ],
                     html::span('inner', $this->gettext('add_category'))
                 )
             );
 
             $p['blocks']['categories']['options']['categories'] = [
                 'content' => html::div('input-group', $new_category->show('') . $add_category),
             ];
 
             $this->rc->output->add_label('delete', 'calendar.remove_category');
             $this->rc->output->add_script('
 function rcube_calendar_add_category() {
     var name = $("#rcmfd_new_category").val();
     if (name.length) {
         var button_label = rcmail.gettext("calendar.remove_category");
         var input = $("<input>").attr({type: "text", name: "_categories[]", size: 30, "class": "form-control"}).val(name);
         var color = $("<input>").attr({type: "text", name: "_colors[]", size: 6, "class": "colors form-control"}).val("000000");
         var button = $("<a>").attr({"class": "button icon delete input-group-text", title: button_label, href: "#rcmfd_new_category"})
             .click(function() { $(this).parent().parent().remove(); })
             .append($("<span>").addClass("inner").text(rcmail.gettext("delete")));
 
         $("<div>").addClass("input-group").append(input).append(color).append($("<span class=\'input-group-append\'>").append(button))
             .appendTo("#calendarcategories");
         color.minicolors(rcmail.env.minicolors_config || {});
         $("#rcmfd_new_category").val("");
     }
 }',
                 'foot'
             );
 
             $this->rc->output->add_script('
 $("#rcmfd_new_category").keypress(function(event) {
     if (event.which == 13) {
         rcube_calendar_add_category();
         event.preventDefault();
     }
 });',
                 'docready'
             );
 
             // load miniColors js/css files
             jqueryui::miniColors();
         }
 
         // virtual birthdays calendar
         if (!isset($no_override['calendar_contact_birthdays'])) {
             $p['blocks']['birthdays']['name'] = $this->gettext('birthdayscalendar');
 
             if (empty($p['current'])) {
                 $p['blocks']['birthdays']['content'] = true;
                 return $p;
             }
 
             $field_id = 'rcmfd_contact_birthdays';
             $input    = new html_checkbox([
                     'name'    => '_contact_birthdays',
                     'id'      => $field_id,
                     'value'   => 1,
                     'onclick' => '$(".calendar_birthday_props").prop("disabled",!this.checked)'
             ]);
 
             $p['blocks']['birthdays']['options']['contact_birthdays'] = [
                 'title'   => html::label($field_id, $this->gettext('displaybirthdayscalendar')),
                 'content' => $input->show($this->rc->config->get('calendar_contact_birthdays') ? 1 : 0),
             ];
 
             $input_attrib = [
                 'class'    => 'calendar_birthday_props',
                 'disabled' => !$this->rc->config->get('calendar_contact_birthdays'),
             ];
 
             $sources  = [];
             $checkbox = new html_checkbox(['name' => '_birthday_adressbooks[]'] + $input_attrib);
 
             foreach ($this->rc->get_address_sources(false, true) as $source) {
                 // Roundcube >= 1.5, Ignore Collected Recipients and Trusted Senders sources
                 if ((defined('rcube_addressbook::TYPE_RECIPIENT') && $source['id'] == (string) rcube_addressbook::TYPE_RECIPIENT)
                     || (defined('rcube_addressbook::TYPE_TRUSTED_SENDER') && $source['id'] == (string) rcube_addressbook::TYPE_TRUSTED_SENDER)
                 ) {
                     continue;
                 }
 
                 $active = in_array($source['id'], (array) $this->rc->config->get('calendar_birthday_adressbooks')) ? $source['id'] : '';
                 $sources[] = html::tag('li', null,
                     html::label(null,
                         $checkbox->show($active, ['value' => $source['id']])
                         . rcube::Q(!empty($source['realname']) ? $source['realname'] : $source['name'])
                     )
                 );
             }
 
             $p['blocks']['birthdays']['options']['birthday_adressbooks'] = [
                 'title'   => rcube::Q($this->gettext('birthdayscalendarsources')),
                 'content' => html::tag('ul', 'proplist', implode("\n", $sources)),
             ];
 
             $field_id = 'rcmfd_birthdays_alarm';
             $select_type = new html_select(['name' => '_birthdays_alarm_type', 'id' => $field_id] + $input_attrib);
             $select_type->add($this->gettext('none'), '');
 
             foreach ($this->driver->alarm_types as $type) {
                 $select_type->add($this->rc->gettext(strtolower("alarm{$type}option"), 'libcalendaring'), $type);
             }
 
             $input_value   = new html_inputfield(['name' => '_birthdays_alarm_value', 'id' => $field_id . 'value', 'size' => 3] + $input_attrib);
             $select_offset = new html_select(['name' => '_birthdays_alarm_offset', 'id' => $field_id . 'offset'] + $input_attrib);
 
             foreach (['-M','-H','-D'] as $trigger) {
                 $select_offset->add($this->rc->gettext('trigger' . $trigger, 'libcalendaring'), $trigger);
             }
 
             $preset      = libcalendaring::parse_alarm_value($this->rc->config->get('calendar_birthdays_alarm_offset', '-1D'));
             $preset_type = $this->rc->config->get('calendar_birthdays_alarm_type', '');
 
             $p['blocks']['birthdays']['options']['birthdays_alarmoffset'] = [
                 'title'   => html::label($field_id, rcube::Q($this->gettext('showalarms'))),
                 'content' => html::div('input-group',
                     $select_type->show($preset_type)
                     . $input_value->show($preset[0]) . ' ' . $select_offset->show($preset[1])
                 ),
             ];
         }
 
         return $p;
     }
 
     /**
      * Handler for preferences_save hook.
      * Executed on Calendar settings form submit.
      *
      * @param array Original parameters
      *
      * @return array Modified parameters
      */
     function preferences_save($p)
     {
         if ($p['section'] == 'calendar') {
             $this->load_driver();
 
             // compose default alarm preset value
             $alarm_offset  = rcube_utils::get_input_value('_alarm_offset', rcube_utils::INPUT_POST);
             $alarm_value   = rcube_utils::get_input_value('_alarm_value', rcube_utils::INPUT_POST);
             $default_alarm = $alarm_offset[0] . intval($alarm_value) . $alarm_offset[1];
 
             $birthdays_alarm_offset = rcube_utils::get_input_value('_birthdays_alarm_offset', rcube_utils::INPUT_POST);
             $birthdays_alarm_value  = rcube_utils::get_input_value('_birthdays_alarm_value', rcube_utils::INPUT_POST);
             $birthdays_alarm_value  = $birthdays_alarm_offset[0] . intval($birthdays_alarm_value) . $birthdays_alarm_offset[1];
 
             $p['prefs'] = [
                 'calendar_default_view' => rcube_utils::get_input_value('_default_view', rcube_utils::INPUT_POST),
                 'calendar_timeslots'    => intval(rcube_utils::get_input_value('_timeslots', rcube_utils::INPUT_POST)),
                 'calendar_first_day'    => intval(rcube_utils::get_input_value('_first_day', rcube_utils::INPUT_POST)),
                 'calendar_first_hour'   => intval(rcube_utils::get_input_value('_first_hour', rcube_utils::INPUT_POST)),
                 'calendar_work_start'   => intval(rcube_utils::get_input_value('_work_start', rcube_utils::INPUT_POST)),
                 'calendar_work_end'     => intval(rcube_utils::get_input_value('_work_end', rcube_utils::INPUT_POST)),
                 'calendar_show_weekno'  => intval(rcube_utils::get_input_value('_show_weekno', rcube_utils::INPUT_POST)),
                 'calendar_event_coloring'       => intval(rcube_utils::get_input_value('_event_coloring', rcube_utils::INPUT_POST)),
                 'calendar_default_alarm_type'   => rcube_utils::get_input_value('_alarm_type', rcube_utils::INPUT_POST),
                 'calendar_default_alarm_offset' => $default_alarm,
                 'calendar_default_calendar'     => rcube_utils::get_input_value('_default_calendar', rcube_utils::INPUT_POST),
                 'calendar_date_format'          => null,  // clear previously saved values
                 'calendar_time_format'          => null,
                 'calendar_contact_birthdays'      => (bool) rcube_utils::get_input_value('_contact_birthdays', rcube_utils::INPUT_POST),
                 'calendar_birthday_adressbooks'   => (array) rcube_utils::get_input_value('_birthday_adressbooks', rcube_utils::INPUT_POST),
                 'calendar_birthdays_alarm_type'   => rcube_utils::get_input_value('_birthdays_alarm_type', rcube_utils::INPUT_POST),
                 'calendar_birthdays_alarm_offset' => $birthdays_alarm_value ?: null,
                 'calendar_itip_after_action'      => intval(rcube_utils::get_input_value('_after_action', rcube_utils::INPUT_POST)),
             ];
 
             if ($p['prefs']['calendar_itip_after_action'] == 4) {
                 $p['prefs']['calendar_itip_after_action'] = rcube_utils::get_input_value('_after_action_folder', rcube_utils::INPUT_POST, true);
             }
 
             // categories
             if (empty($this->driver->nocategories)) {
                 $old_categories = $new_categories = [];
 
                 foreach ($this->driver->list_categories() as $name => $color) {
                     $old_categories[md5($name)] = $name;
                 }
 
                 $categories = (array) rcube_utils::get_input_value('_categories', rcube_utils::INPUT_POST);
                 $colors     = (array) rcube_utils::get_input_value('_colors', rcube_utils::INPUT_POST);
 
                 foreach ($categories as $key => $name) {
                     if (!isset($colors[$key])) {
                         continue;
                     }
 
                     $color = preg_replace('/^#/', '', strval($colors[$key]));
 
                     // rename categories in existing events -> driver's job
                     if (!empty($old_categories[$key])) {
                         $oldname = $old_categories[$key];
                         $this->driver->replace_category($oldname, $name, $color);
                         unset($old_categories[$key]);
                     }
                     else {
                         $this->driver->add_category($name, $color);
                     }
 
                     $new_categories[$name] = $color;
                 }
 
                 // these old categories have been removed, alter events accordingly -> driver's job
                 foreach ((array) $old_categories as $key => $name) {
                     $this->driver->remove_category($name);
                 }
 
                 $p['prefs']['calendar_categories'] = $new_categories;
             }
         }
 
         return $p;
     }
 
     /**
      * Dispatcher for calendar actions initiated by the client
      */
     function calendar_action()
     {
         $action  = rcube_utils::get_input_value('action', rcube_utils::INPUT_GPC);
         $cal     = rcube_utils::get_input_value('c', rcube_utils::INPUT_GPC);
         $success = false;
         $reload  = false;
 
         if (isset($cal['showalarms'])) {
             $cal['showalarms'] = intval($cal['showalarms']);
         }
 
         switch ($action) {
         case "form-new":
         case "form-edit":
             echo $this->ui->calendar_editform($action, $cal);
             exit;
 
         case "new":
             $success = $this->driver->create_calendar($cal);
             $reload  = true;
             break;
 
         case "edit":
             $success = $this->driver->edit_calendar($cal);
             $reload  = true;
             break;
 
         case "delete":
             if ($success = $this->driver->delete_calendar($cal)) {
                 $this->rc->output->command('plugin.destroy_source', ['id' => $cal['id']]);
             }
             break;
 
         case "subscribe":
             if (!$this->driver->subscribe_calendar($cal)) {
                 $this->rc->output->show_message($this->gettext('errorsaving'), 'error');
             }
             else {
                 $calendars = $this->driver->list_calendars();
                 $calendar  = !empty($calendars[$cal['id']]) ? $calendars[$cal['id']] : null;
 
                 // find parent folder and check if it's a "user calendar"
                 // if it's also activated we need to refresh it (#5340)
                 while (!empty($calendar['parent'])) {
                     if (isset($calendars[$calendar['parent']])) {
                         $calendar = $calendars[$calendar['parent']];
                     }
                     else {
                         break;
                     }
                 }
 
                 if ($calendar && $calendar['id'] != $cal['id']
                     && !empty($calendar['active'])
                     && $calendar['group'] == "other user"
                 ) {
                     $this->rc->output->command('plugin.refresh_source', $calendar['id']);
                 }
             }
             return;
 
         case "search":
             $results    = [];
             $color_mode = $this->rc->config->get('calendar_event_coloring', $this->defaults['calendar_event_coloring']);
             $query      = rcube_utils::get_input_value('q', rcube_utils::INPUT_GPC);
             $source     = rcube_utils::get_input_value('source', rcube_utils::INPUT_GPC);
 
             foreach ((array) $this->driver->search_calendars($query, $source) as $id => $prop) {
                 $editname = $prop['editname'];
                 unset($prop['editname']);  // force full name to be displayed
                 $prop['active'] = false;
 
                 // let the UI generate HTML and CSS representation for this calendar
                 $html = $this->ui->calendar_list_item($id, $prop, $jsenv);
                 $cal  = $jsenv[$id];
                 $cal['editname'] = $editname;
                 $cal['html']     = $html;
 
                 if (!empty($prop['color'])) {
                     $cal['css'] = $this->ui->calendar_css_classes($id, $prop, $color_mode);
                 }
 
                 $results[] = $cal;
             }
 
             // report more results available
             if (!empty($this->driver->search_more_results)) {
                 $this->rc->output->show_message('autocompletemore', 'notice');
             }
 
             $reqid = rcube_utils::get_input_value('_reqid', rcube_utils::INPUT_GPC);
             $this->rc->output->command('multi_thread_http_response', $results, $reqid);
             return;
         }
 
         if ($success) {
             $this->rc->output->show_message('successfullysaved', 'confirmation');
         }
         else {
             $error_msg = $this->gettext('errorsaving');
             if (!empty($this->driver->last_error)) {
                 $error_msg .= ': ' . $this->driver->last_error;
             }
             $this->rc->output->show_message($error_msg, 'error');
         }
 
         $this->rc->output->command('plugin.unlock_saving');
 
         if ($success && $reload) {
             $this->rc->output->command('plugin.reload_view');
         }
     }
 
     /**
      * Dispatcher for event actions initiated by the client
      */
     function event_action()
     {
         $action  = rcube_utils::get_input_value('action', rcube_utils::INPUT_GPC);
         $event   = rcube_utils::get_input_value('e', rcube_utils::INPUT_POST, true);
         $success = $reload = $got_msg = false;
         $old     = null;
 
         // read old event data in order to find changes
         if ((!empty($event['_notify']) || !empty($event['_decline'])) && $action != 'new') {
             $old = $this->driver->get_event($event);
 
             // load main event if savemode is 'all' or if deleting 'future' events
             if (!empty($old['recurrence_id'])
                 && !empty($event['_savemode'])
                 && ($event['_savemode'] == 'all' || ($event['_savemode'] == 'future' && $action == 'remove' && empty($event['_decline'])))
             ) {
                 $old['id'] = $old['recurrence_id'];
                 $old = $this->driver->get_event($old);
             }
         }
 
         switch ($action) {
         case "new":
             // create UID for new event
             $event['uid'] = $this->generate_uid();
             if (!$this->write_preprocess($event, $action)) {
                 $got_msg = true;
             }
             else if ($success = $this->driver->new_event($event)) {
                 $event['id']        = $event['uid'];
                 $event['_savemode'] = 'all';
 
                 $this->cleanup_event($event);
                 $this->event_save_success($event, null, $action, true);
                 $this->talk_room_update($event);
             }
 
             $reload = $success && !empty($event['recurrence']) ? 2 : 1;
             break;
 
         case "edit":
             if (!$this->write_preprocess($event, $action)) {
                 $got_msg = true;
             }
             else if ($success = $this->driver->edit_event($event)) {
                 $this->cleanup_event($event);
                 $this->event_save_success($event, $old, $action, $success);
                 $this->talk_room_update($event);
             }
 
             $reload = $success && (!empty($event['recurrence']) || !empty($event['_savemode']) || !empty($event['_fromcalendar'])) ? 2 : 1;
             break;
 
         case "resize":
             if (!$this->write_preprocess($event, $action)) {
                 $got_msg = true;
             }
             else if ($success = $this->driver->resize_event($event)) {
                 $this->event_save_success($event, $old, $action, $success);
             }
 
             $reload = !empty($event['_savemode']) ? 2 : 1;
             break;
 
         case "move":
             if (!$this->write_preprocess($event, $action)) {
                 $got_msg = true;
             }
             else if ($success = $this->driver->move_event($event)) {
                 $this->event_save_success($event, $old, $action, $success);
             }
 
             $reload = $success && !empty($event['_savemode']) ? 2 : 1;
             break;
 
         case "remove":
             // remove previous deletes
             $undo_time = $this->driver->undelete ? $this->rc->config->get('undo_timeout', 0) : 0;
 
             // search for event if only UID is given
             if (!isset($event['calendar']) && !empty($event['uid'])) {
                 if (!($event = $this->driver->get_event($event, calendar_driver::FILTER_WRITEABLE))) {
                     break;
                 }
                 $undo_time = 0;
             }
 
             // Note: the driver is responsible for setting $_SESSION['calendar_event_undo']
             //       containing 'ts' and 'data' elements
             $success = $this->driver->remove_event($event, $undo_time < 1);
             $reload = (!$success || !empty($event['_savemode'])) ? 2 : 1;
 
             if ($undo_time > 0 && $success) {
                 // display message with Undo link.
                 $onclick = sprintf("%s.http_request('event', 'action=undo', %s.display_message('', 'loading'))",
                     rcmail_output::JS_OBJECT_NAME,
                     rcmail_output::JS_OBJECT_NAME
                 );
                 $msg = html::span(null, $this->gettext('successremoval'))
                     . ' ' . html::a(['onclick' => $onclick], $this->gettext('undo'));
 
                 $this->rc->output->show_message($msg, 'confirmation', null, true, $undo_time);
                 $got_msg = true;
             }
             else if ($success) {
                 $this->rc->output->show_message('calendar.successremoval', 'confirmation');
                 $got_msg = true;
             }
 
             // send cancellation for the main event
             if (isset($event['_savemode']) && $event['_savemode'] == 'all') {
                 unset($old['_instance'], $old['recurrence_date'], $old['recurrence_id']);
             }
             // send an update for the main event's recurrence rule instead of a cancellation message
             else if (isset($event['_savemode']) && $event['_savemode'] == 'future' && !is_bool($success)) {
                 $event['_savemode'] = 'all';  // force event_save_success() to load master event
                 $action  = 'edit';
                 $success = true;
             }
 
             // send iTIP reply that participant has declined the event
             if ($success && !empty($event['_decline'])) {
                 $emails    = $this->get_user_emails();
                 $organizer = null;
 
                 foreach ($old['attendees'] as $i => $attendee) {
                     if ($attendee['role'] == 'ORGANIZER') {
                         $organizer = $attendee;
                     }
                     else if (!empty($attendee['email']) && in_array(strtolower($attendee['email']), $emails)) {
                         $old['attendees'][$i]['status'] = 'DECLINED';
                         $reply_sender = $attendee['email'];
                     }
                 }
 
                 if ($event['_savemode'] == 'future' && $event['id'] != $old['id']) {
                     $old['thisandfuture'] = true;
                 }
 
                 $itip = $this->load_itip();
                 $itip->set_sender_email($reply_sender);
 
                 if ($organizer && $itip->send_itip_message($old, 'REPLY', $organizer, 'itipsubjectdeclined', 'itipmailbodydeclined')) {
                     $mailto = !empty($organizer['name']) ? $organizer['name'] : $organizer['email'];
                     $msg    = $this->gettext(['name' => 'sentresponseto', 'vars' => ['mailto' => $mailto]]);
 
                     $this->rc->output->command('display_message', $msg, 'confirmation');
                 }
                 else {
                     $this->rc->output->command('display_message', $this->gettext('itipresponseerror'), 'error');
                 }
             }
             else if ($success) {
                 $this->event_save_success($event, $old, $action, $success);
             }
 
             break;
 
         case "undo":
             // Restore deleted event
             if (!empty($_SESSION['calendar_event_undo']['data'])) {
                 $event   = $_SESSION['calendar_event_undo']['data'];
                 $success = $this->driver->restore_event($event);
             }
 
             if ($success) {
                 $this->rc->session->remove('calendar_event_undo');
                 $this->rc->output->show_message('calendar.successrestore', 'confirmation');
                 $got_msg = true;
                 $reload  = 2;
             }
 
             break;
 
         case "rsvp":
             $itip_sending  = $this->rc->config->get('calendar_itip_send_option', $this->defaults['calendar_itip_send_option']);
             $status        = rcube_utils::get_input_value('status', rcube_utils::INPUT_POST);
             $attendees     = rcube_utils::get_input_value('attendees', rcube_utils::INPUT_POST);
             $reply_comment = $event['comment'];
 
             $this->write_preprocess($event, 'edit');
             $ev = $this->driver->get_event($event);
             $ev['attendees'] = $event['attendees'];
             $ev['free_busy'] = $event['free_busy'];
             $ev['_savemode'] = $event['_savemode'];
             $ev['comment']   = $reply_comment;
 
             // send invitation to delegatee + add it as attendee
             if ($status == 'delegated' && !empty($event['to'])) {
                 $itip = $this->load_itip();
                 if ($itip->delegate_to($ev, $event['to'], !empty($event['rsvp']), $attendees)) {
                     $this->rc->output->show_message('calendar.itipsendsuccess', 'confirmation');
                     $noreply = false;
                 }
             }
 
             $event = $ev;
 
             // compose a list of attendees affected by this change
             $updated_attendees = array_filter(array_map(function($j) use ($event) {
                     return $event['attendees'][$j];
                 },
                 $attendees
             ));
 
             if ($success = $this->driver->edit_rsvp($event, $status, $updated_attendees)) {
                 $noreply = rcube_utils::get_input_value('noreply', rcube_utils::INPUT_GPC);
                 $noreply = intval($noreply) || $status == 'needs-action' || $itip_sending === 0;
                 $reload  = $event['calendar'] != $ev['calendar'] || !empty($event['recurrence']) ? 2 : 1;
                 $emails  = $this->get_user_emails();
                 $ownedResourceEmails = $this->owned_resources_emails();
                 $organizer = null;
                 $resourceConfirmation = false;
 
                 foreach ($event['attendees'] as $i => $attendee) {
                     if ($attendee['role'] == 'ORGANIZER') {
                         $organizer = $attendee;
                     }
                     else if (!empty($attendee['email']) && in_array_nocase($attendee['email'], $emails)) {
                         $reply_sender = $attendee['email'];
                     }
                     else if (!empty($attendee['cutype']) && $attendee['cutype'] == 'RESOURCE' && !empty($attendee['email']) && in_array_nocase($attendee['email'], $ownedResourceEmails)) {
                         $resourceConfirmation = true;
                         // Note on behalf of which resource this update is going to be sent out
                         $event['_resource'] = $attendee['email'];
                     }
                 }
 
                 if (!$noreply) {
                     $itip = $this->load_itip();
                     $itip->set_sender_email($reply_sender);
                     $event['thisandfuture'] = $event['_savemode'] == 'future';
                     $bodytextprefix = $resourceConfirmation ? 'itipmailbodyresource' : 'itipmailbody';
 
                     if ($organizer && $itip->send_itip_message($event, 'REPLY', $organizer, 'itipsubject' . $status, $bodytextprefix . $status)) {
                         $mailto = !empty($organizer['name']) ? $organizer['name'] : $organizer['email'];
                         $msg    = $this->gettext(['name' => 'sentresponseto', 'vars' => ['mailto' => $mailto]]);
 
                         $this->rc->output->command('display_message', $msg, 'confirmation');
                     }
                     else {
                         $this->rc->output->command('display_message', $this->gettext('itipresponseerror'), 'error');
                     }
                 }
 
                 // refresh all calendars
                 if ($event['calendar'] != $ev['calendar']) {
                     $this->rc->output->command('plugin.refresh_calendar', ['source' => null, 'refetch' => true]);
                     $reload = 0;
                 }
             }
 
             break;
 
         case "dismiss":
             $event['ids'] = explode(',', $event['id']);
             $plugin  = $this->rc->plugins->exec_hook('dismiss_alarms', $event);
             $success = $plugin['success'];
 
             foreach ($event['ids'] as $id) {
                 if (strpos($id, 'cal:') === 0) {
                     $success |= $this->driver->dismiss_alarm(substr($id, 4), $event['snooze']);
                 }
             }
 
             break;
 
         case "changelog":
             $data = $this->driver->get_event_changelog($event);
             if (is_array($data) && !empty($data)) {
                 $lib = $this->lib;
                 $dtformat = $this->rc->config->get('date_format') . ' ' . $this->rc->config->get('time_format');
                 array_walk($data, function(&$change) use ($lib, $dtformat) {
                     if (!empty($change['date'])) {
                         $dt = $lib->adjust_timezone($change['date']);
 
                         if ($dt instanceof DateTimeInterface) {
                             $change['date'] = $this->rc->format_date($dt, $dtformat, false);
                         }
                     }
                 });
 
                 $this->rc->output->command('plugin.render_event_changelog', $data);
             }
             else {
                 $this->rc->output->command('plugin.render_event_changelog', false);
             }
 
             $got_msg = true;
             $reload  = false;
 
             break;
 
         case "diff":
             $data = $this->driver->get_event_diff($event, $event['rev1'], $event['rev2']);
             if (is_array($data)) {
                 // convert some properties, similar to self::_client_event()
                 $lib = $this->lib;
                 array_walk($data['changes'], function(&$change, $i) use ($event, $lib) {
                     // convert date cols
                     foreach (['start', 'end', 'created', 'changed'] as $col) {
                         if ($change['property'] == $col) {
                             $change['old'] = $lib->adjust_timezone($change['old'], strlen($change['old']) == 10)->format('c');
                             $change['new'] = $lib->adjust_timezone($change['new'], strlen($change['new']) == 10)->format('c');
                         }
                     }
                     // create textual representation for alarms and recurrence
                     if ($change['property'] == 'alarms') {
                         if (is_array($change['old'])) {
                             $change['old_'] = libcalendaring::alarm_text($change['old']);
                         }
                         if (is_array($change['new'])) {
                             $change['new_'] = libcalendaring::alarm_text(array_merge((array)$change['old'], $change['new']));
                         }
                     }
                     if ($change['property'] == 'recurrence') {
                         if (is_array($change['old'])) {
                             $change['old_'] = $lib->recurrence_text($change['old']);
                         }
                         if (is_array($change['new'])) {
                             $change['new_'] = $lib->recurrence_text(array_merge((array)$change['old'], $change['new']));
                         }
                     }
                     if ($change['property'] == 'attachments') {
                         if (is_array($change['old'])) {
                             $change['old']['classname'] = rcube_utils::file2class($change['old']['mimetype'], $change['old']['name']);
                         }
                         if (is_array($change['new'])) {
                             $change['new']['classname'] = rcube_utils::file2class($change['new']['mimetype'], $change['new']['name']);
                         }
                     }
                     // compute a nice diff of description texts
                     if ($change['property'] == 'description') {
                         $change['diff_'] = libkolab::html_diff($change['old'], $change['new']);
                     }
                 });
 
                 $this->rc->output->command('plugin.event_show_diff', $data);
             }
             else {
                 $this->rc->output->command('display_message', $this->gettext('objectdiffnotavailable'), 'error');
             }
 
             $got_msg = true;
             $reload  = false;
 
             break;
 
         case "show":
             if ($event = $this->driver->get_event_revison($event, $event['rev'])) {
                 $this->rc->output->command('plugin.event_show_revision', $this->_client_event($event));
             }
             else {
                 $this->rc->output->command('display_message', $this->gettext('objectnotfound'), 'error');
             }
 
             $got_msg = true;
             $reload  = false;
             break;
 
         case "restore":
             if ($success = $this->driver->restore_event_revision($event, $event['rev'])) {
                 $_event = $this->driver->get_event($event);
                 $reload = $_event['recurrence'] ? 2 : 1;
                 $msg = $this->gettext(['name' => 'objectrestoresuccess', 'vars' => ['rev' => $event['rev']]]);
                 $this->rc->output->command('display_message', $msg, 'confirmation');
                 $this->rc->output->command('plugin.close_history_dialog');
             }
             else {
                 $this->rc->output->command('display_message', $this->gettext('objectrestoreerror'), 'error');
                 $reload = 0;
             }
 
             $got_msg = true;
             break;
         }
 
         // show confirmation/error message
         if (!$got_msg) {
             if ($success) {
                 $this->rc->output->show_message('successfullysaved', 'confirmation');
             }
             else {
                 $this->rc->output->show_message('calendar.errorsaving', 'error');
             }
         }
 
         // unlock client
         $this->rc->output->command('plugin.unlock_saving', $success);
 
         // update event object on the client or trigger a complete refresh if too complicated
         if ($reload && empty($_REQUEST['_framed'])) {
             $args = ['source' => $event['calendar']];
             if ($reload > 1) {
                 $args['refetch'] = true;
             }
             else if ($success && $action != 'remove') {
                 $args['update'] = $this->_client_event($this->driver->get_event($event), true);
             }
             $this->rc->output->command('plugin.refresh_calendar', $args);
         }
     }
 
     /**
      * Helper method sending iTip notifications after successful event updates
      */
     private function event_save_success(&$event, $old, $action, $success)
     {
         // $success is a new event ID
         if ($success !== true) {
             // send update notification on the main event
             if (!empty($event['_savemode']) && $event['_savemode'] == 'future' && !empty($event['_notify'])
                 && !empty($old['attendees']) && !empty($old['recurrence_id'])
             ) {
                 $master = $this->driver->get_event(['id' => $old['recurrence_id'], 'calendar' => $old['calendar']], 0, true);
                 unset($master['_instance'], $master['recurrence_date']);
 
                 $sent = $this->notify_attendees($master, null, $action, $event['_comment'], false);
                 if ($sent < 0) {
                     $this->rc->output->show_message('calendar.errornotifying', 'error');
                 }
 
                 $event['attendees'] = $master['attendees'];  // this tricks us into the next if clause
             }
 
             // delete old reference if saved as new
             if (!empty($event['_savemode']) && ($event['_savemode'] == 'future' || $event['_savemode'] == 'new')) {
                 $old = null;
             }
 
             $event['id']        = $success;
             $event['_savemode'] = 'all';
         }
 
         // send out notifications
         if (!empty($event['_notify']) && (!empty($event['attendees']) || !empty($old['attendees']))) {
             $_savemode = $event['_savemode'] ?? null;
 
             // send notification for the main event when savemode is 'all'
             if ($action != 'remove' && $_savemode == 'all'
                 && (!empty($event['recurrence_id']) || !empty($old['recurrence_id']) || ($old && $old['id'] != $event['id']))
             ) {
                 if (!empty($event['recurrence_id'])) {
                     $event['id'] = $event['recurrence_id'];
                 }
                 else if (!empty($old['recurrence_id'])) {
                     $event['id'] = $old['recurrence_id'];
                 }
                 else {
                     $event['id'] = $old['id'];
                 }
                 $event = $this->driver->get_event($event, 0, true);
                 unset($event['_instance'], $event['recurrence_date']);
             }
             else {
                 // make sure we have the complete record
                 $event = $action == 'remove' ? $old : $this->driver->get_event($event, 0, true);
             }
 
             $event['_savemode'] = $_savemode;
 
             if ($old) {
                 $old['thisandfuture'] = $_savemode == 'future';
             }
 
             // only notify if data really changed (TODO: do diff check on client already)
             if (!$old || $action == 'remove' || self::event_diff($event, $old)) {
                 $comment = isset($event['_comment']) ? $event['_comment'] : null;
                 $sent    = $this->notify_attendees($event, $old, $action, $comment);
 
                 if ($sent > 0) {
                     $this->rc->output->show_message('calendar.itipsendsuccess', 'confirmation');
                 }
                 else if ($sent < 0) {
                     $this->rc->output->show_message('calendar.errornotifying', 'error');
                 }
             }
         }
     }
 
     /**
      * Handler for load-requests from fullcalendar
      * This will return pure JSON formatted output
      */
     function load_events()
     {
         $start  = $this->input_timestamp('start', rcube_utils::INPUT_GET);
         $end    = $this->input_timestamp('end', rcube_utils::INPUT_GET);
         $query  = rcube_utils::get_input_value('q', rcube_utils::INPUT_GET);
         $source = rcube_utils::get_input_value('source', rcube_utils::INPUT_GET);
 
         $events = $this->driver->load_events($start, $end, $query, $source);
         echo $this->encode($events, !empty($query));
         exit;
     }
 
     /**
      * Handler for requests fetching event counts for calendars
      */
     public function count_events()
     {
         // don't update session on these requests (avoiding race conditions)
         $this->rc->session->nowrite = true;
 
         $start  = rcube_utils::get_input_value('start', rcube_utils::INPUT_GET);
         $source = rcube_utils::get_input_value('source', rcube_utils::INPUT_GET);
         $end    = rcube_utils::get_input_value('end', rcube_utils::INPUT_GET);
 
         if (!$start) {
             $start = new DateTime('today 00:00:00', $this->timezone);
             $start = $start->format('U');
         }
 
         $counts = $this->driver->count_events($source, $start, $end);
 
         $this->rc->output->command('plugin.update_counts', ['counts' => $counts]);
     }
 
     /**
      * Load event data from an iTip message attachment
      */
     public function itip_events($msgref)
     {
         $path = explode('/', $msgref);
         $msg  = array_pop($path);
         $mbox = join('/', $path);
         list($uid, $mime_id) = explode('#', $msg);
         $events = [];
 
         if ($event = $this->lib->mail_get_itip_object($mbox, $uid, $mime_id, 'event')) {
             $partstat = 'NEEDS-ACTION';
 
             $event['id']        = $event['uid'];
             $event['temporary'] = true;
             $event['readonly']  = true;
             $event['calendar']  = '--invitation--itip';
             $event['className'] = 'fc-invitation-' . strtolower($partstat);
             $event['_mbox']     = $mbox;
             $event['_uid']      = $uid;
             $event['_part']     = $mime_id;
 
             $events[] = $this->_client_event($event, true);
 
             // add recurring instances
             if (!empty($event['recurrence'])) {
                 // Some installations can't handle all occurrences (aborting the request w/o an error in log)
                 $freq = !empty($event['recurrence']['FREQ']) ? $event['recurrence']['FREQ'] : null;
                 $end  = clone $event['start'];
                 $end->add(new DateInterval($freq == 'DAILY' ? 'P1Y' : 'P10Y'));
 
                 foreach ($this->driver->get_recurring_events($event, $event['start'], $end) as $recurring) {
                     $recurring['temporary'] = true;
                     $recurring['readonly']  = true;
                     $recurring['calendar']  = '--invitation--itip';
 
                     $events[] = $this->_client_event($recurring, true);
                 }
             }
         }
 
         return $events;
     }
 
     /**
      * Handler for keep-alive requests
      * This will check for updated data in active calendars and sync them to the client
      */
     public function refresh($attr)
     {
         // refresh the entire calendar every 10th time to also sync deleted events
         if (rand(0, 10) == 10) {
             $this->rc->output->command('plugin.refresh_calendar', ['refetch' => true]);
             return;
         }
 
         $counts = [];
 
         foreach ($this->driver->list_calendars(calendar_driver::FILTER_ACTIVE) as $cal) {
             $events = $this->driver->load_events(
                 rcube_utils::get_input_value('start', rcube_utils::INPUT_GPC),
                 rcube_utils::get_input_value('end', rcube_utils::INPUT_GPC),
                 rcube_utils::get_input_value('q', rcube_utils::INPUT_GPC),
                 $cal['id'],
                 1,
                 $attr['last']
             );
 
             foreach ($events as $event) {
                 $this->rc->output->command(
                     'plugin.refresh_calendar',
                     ['source' => $cal['id'], 'update' => $this->_client_event($event)]
                 );
             }
 
             // refresh count for this calendar
             if (!empty($cal['counts'])) {
                 $today = new DateTime('today 00:00:00', $this->timezone);
                 $counts += $this->driver->count_events($cal['id'], $today->format('U'));
             }
         }
 
         if (!empty($counts)) {
             $this->rc->output->command('plugin.update_counts', ['counts' => $counts]);
         }
     }
 
     /**
      * Handler for pending_alarms plugin hook triggered by the calendar module on keep-alive requests.
      * This will check for pending notifications and pass them to the client
      */
     public function pending_alarms($p)
     {
         $this->load_driver();
 
         $time = !empty($p['time']) ? $p['time'] : time();
 
         if ($alarms = $this->driver->pending_alarms($time)) {
             foreach ($alarms as $alarm) {
                 $alarm['id'] = 'cal:' . $alarm['id'];  // prefix ID with cal:
                 $p['alarms'][] = $alarm;
             }
         }
 
         // get alarms for birthdays calendar
         if (
             $this->rc->config->get('calendar_contact_birthdays')
             && $this->rc->config->get('calendar_birthdays_alarm_type') == 'DISPLAY'
         ) {
             $cache = $this->rc->get_cache('calendar.birthdayalarms', 'db');
 
             foreach ($this->driver->load_birthday_events($time, $time + 86400 * 60) as $e) {
                 $alarm = libcalendaring::get_next_alarm($e);
 
                 // overwrite alarm time with snooze value (or null if dismissed)
                 if ($dismissed = $cache->get($e['id'])) {
                     $alarm['time'] = $dismissed['notifyat'];
                 }
 
                 // add to list if alarm is set
                 if ($alarm && !empty($alarm['time']) && $alarm['time'] <= $time) {
                     $e['id']       = 'cal:bday:' . $e['id'];
                     $e['notifyat'] = $alarm['time'];
                     $p['alarms'][] = $e;
                 }
             }
         }
 
         return $p;
     }
 
     /**
      * Handler for alarm dismiss hook triggered by libcalendaring
      */
     public function dismiss_alarms($p)
     {
         $this->load_driver();
 
         foreach ((array) $p['ids'] as $id) {
             if (strpos($id, 'cal:bday:') === 0) {
                 $p['success'] |= $this->driver->dismiss_birthday_alarm(substr($id, 9), $p['snooze']);
             }
             else if (strpos($id, 'cal:') === 0) {
                 $p['success'] |= $this->driver->dismiss_alarm(substr($id, 4), $p['snooze']);
             }
         }
 
         return $p;
     }
 
     /**
      * Handler for check-recent requests which are accidentally sent to calendar
      */
     function check_recent()
     {
         // NOP
         $this->rc->output->send();
     }
 
     /**
      * Hook triggered when a contact is saved
      */
     function contact_update($p)
     {
         // clear birthdays calendar cache
         if (!empty($p['record']['birthday'])) {
             $cache = $this->rc->get_cache('calendar.birthdays', 'db');
             $cache->remove();
         }
     }
 
     /**
      *
      */
     function import_events()
     {
         // Upload progress update
         if (!empty($_GET['_progress'])) {
             $this->rc->upload_progress();
         }
 
         @set_time_limit(0);
 
         // process uploaded file if there is no error
         $err = $_FILES['_data']['error'];
 
         if (!$err && !empty($_FILES['_data']['tmp_name'])) {
             $calendar   = rcube_utils::get_input_value('calendar', rcube_utils::INPUT_GPC);
             $rangestart = !empty($_REQUEST['_range']) ? date_create("now -" . intval($_REQUEST['_range']) . " months") : 0;
 
             // extract zip file
             if ($_FILES['_data']['type'] == 'application/zip') {
                 $count = 0;
                 if (class_exists('ZipArchive', false)) {
                     $zip = new ZipArchive();
                     if ($zip->open($_FILES['_data']['tmp_name'])) {
                         $randname = uniqid('zip-' . session_id(), true);
                         $tmpdir = slashify($this->rc->config->get('temp_dir', sys_get_temp_dir())) . $randname;
                         mkdir($tmpdir, 0700);
 
                         // extract each ical file from the archive and import it
                         for ($i = 0; $i < $zip->numFiles; $i++) {
                             $filename = $zip->getNameIndex($i);
                             if (preg_match('/\.ics$/i', $filename)) {
                                 $tmpfile = $tmpdir . '/' . basename($filename);
                                 if (copy('zip://' . $_FILES['_data']['tmp_name'] . '#'.$filename, $tmpfile)) {
                                     $count += $this->import_from_file($tmpfile, $calendar, $rangestart, $errors);
                                     unlink($tmpfile);
                                 }
                             }
                         }
 
                         rmdir($tmpdir);
                         $zip->close();
                     }
                     else {
                         $errors = 1;
                         $msg = 'Failed to open zip file.';
                     }
                 }
                 else {
                     $errors = 1;
                     $msg = 'Zip files are not supported for import.';
                 }
             }
             else {
                 // attempt to import teh uploaded file directly
                 $count = $this->import_from_file($_FILES['_data']['tmp_name'], $calendar, $rangestart, $errors);
             }
 
             if ($count) {
                 $this->rc->output->command('display_message', $this->gettext(['name' => 'importsuccess', 'vars' => ['nr' => $count]]), 'confirmation');
                 $this->rc->output->command('plugin.import_success', ['source' => $calendar, 'refetch' => true]);
             }
             else if (!$errors) {
                 $this->rc->output->command('display_message', $this->gettext('importnone'), 'notice');
                 $this->rc->output->command('plugin.import_success', ['source' => $calendar]);
             }
             else {
                 $this->rc->output->command('plugin.import_error', ['message' => $this->gettext('importerror') . ($msg ? ': ' . $msg : '')]);
             }
         }
         else {
             if ($err == UPLOAD_ERR_INI_SIZE || $err == UPLOAD_ERR_FORM_SIZE) {
                 $max = $this->rc->show_bytes(parse_bytes(ini_get('upload_max_filesize')));
                 $msg = $this->rc->gettext(['name' => 'filesizeerror', 'vars' => ['size' => $max]]);
             }
             else {
                 $msg = $this->rc->gettext('fileuploaderror');
             }
 
             $this->rc->output->command('plugin.import_error', ['message' => $msg]);
         }
 
         $this->rc->output->send('iframe');
     }
 
     /**
      * Helper function to parse and import a single .ics file
      */
     private function import_from_file($filepath, $calendar, $rangestart, &$errors)
     {
         $user_email = $this->rc->user->get_username();
         $ical       = $this->get_ical();
         $errors     = !$ical->fopen($filepath);
 
         $count = $i = 0;
 
         foreach ($ical as $event) {
             // keep the browser connection alive on long import jobs
             if (++$i > 100 && $i % 100 == 0) {
                 echo "<!-- -->";
                 ob_flush();
             }
 
             // TODO: correctly handle recurring events which start before $rangestart
             if ($rangestart && $event['end'] < $rangestart
                 && (empty($event['recurrence']) || (!empty($event['recurrence']['until']) && $event['recurrence']['until'] < $rangestart))
             ) {
                 continue;
             }
 
             $event['_owner']   = $user_email;
             $event['calendar'] = $calendar;
 
             if ($this->driver->new_event($event)) {
                 $count++;
             }
             else {
                 $errors++;
             }
         }
 
         return $count;
     }
 
     /**
      * Construct the ics file for exporting events to iCalendar format;
      */
     function export_events($terminate = true)
     {
         $start       = rcube_utils::get_input_value('start', rcube_utils::INPUT_GET);
         $end         = rcube_utils::get_input_value('end', rcube_utils::INPUT_GET);
         $event_id    = rcube_utils::get_input_value('id', rcube_utils::INPUT_GET);
         $attachments = rcube_utils::get_input_value('attachments', rcube_utils::INPUT_GET);
         $calid       = rcube_utils::get_input_value('source', rcube_utils::INPUT_GET);
 
         if (!isset($start)) {
             $start = 'today -1 year';
         }
         if (!is_numeric($start)) {
             $start = strtotime($start . ' 00:00:00');
         }
         if (!$end) {
             $end = 'today +10 years';
         }
         if (!is_numeric($end)) {
             $end = strtotime($end . ' 23:59:59');
         }
 
         $filename  = $calid;
         $calendars = $this->driver->list_calendars();
         $events    = [];
 
         if (!empty($calendars[$calid])) {
             $filename = !empty($calendars[$calid]['name']) ? $calendars[$calid]['name'] : $calid;
             $filename = asciiwords(html_entity_decode($filename));  // to 7bit ascii
 
             if (!empty($event_id)) {
                 if ($event = $this->driver->get_event(['calendar' => $calid, 'id' => $event_id], 0, true)) {
                     if (!empty($event['recurrence_id'])) {
                         $event = $this->driver->get_event(['calendar' => $calid, 'id' => $event['recurrence_id']], 0, true);
                     }
 
                     $events   = [$event];
                     $filename = asciiwords($event['title']);
 
                     if (empty($filename)) {
                         $filename = 'event';
                     }
                 }
             }
             else {
                 $events = $this->driver->load_events($start, $end, null, $calid, 0);
                 if (empty($filename)) {
                     $filename = $calid;
                 }
             }
         }
 
         header("Content-Type: text/calendar");
         header("Content-Disposition: inline; filename=".$filename.'.ics');
 
         $this->get_ical()->export($events, '', true, $attachments ? [$this->driver, 'get_attachment_body'] : null);
 
         if ($terminate) {
             exit;
         }
     }
 
     /**
      * Handler for iCal feed requests
      */
     function ical_feed_export()
     {
         $session_exists = !empty($_SESSION['user_id']);
 
         // process HTTP auth info
         if (!empty($_SERVER['PHP_AUTH_USER']) && isset($_SERVER['PHP_AUTH_PW'])) {
             $_POST['_user'] = $_SERVER['PHP_AUTH_USER']; // used for rcmail::autoselect_host()
             $auth = $this->rc->plugins->exec_hook('authenticate', [
                 'host' => $this->rc->autoselect_host(),
                 'user' => trim($_SERVER['PHP_AUTH_USER']),
                 'pass' => $_SERVER['PHP_AUTH_PW'],
                 'cookiecheck' => true,
                 'valid'       => true,
             ]);
 
             if ($auth['valid'] && !$auth['abort']) {
                 $this->rc->login($auth['user'], $auth['pass'], $auth['host']);
             }
         }
 
         // require HTTP auth
         if (empty($_SESSION['user_id'])) {
             header('WWW-Authenticate: Basic realm="Kolab Calendar"');
             header('HTTP/1.0 401 Unauthorized');
             exit;
         }
 
         // decode calendar feed hash
         $format  = 'ics';
         $calhash = rcube_utils::get_input_value('_cal', rcube_utils::INPUT_GET);
 
         if (preg_match(($suff_regex = '/\.([a-z0-9]{3,5})$/i'), $calhash, $m)) {
             $format  = strtolower($m[1]);
             $calhash = preg_replace($suff_regex, '', $calhash);
         }
 
         if (!strpos($calhash, ':')) {
             $calhash = base64_decode($calhash);
         }
 
         list($user, $_GET['source']) = explode(':', $calhash, 2);
 
         // sanity check user
         if ($this->rc->user->get_username() == $user) {
             $this->setup();
             $this->load_driver();
             $this->export_events(false);
         }
         else {
             header('HTTP/1.0 404 Not Found');
         }
 
         // don't save session data
         if (!$session_exists) {
             session_destroy();
         }
 
         exit;
     }
 
     /**
      *
      */
     function load_settings()
     {
         $this->lib->load_settings();
         $this->defaults += $this->lib->defaults;
 
         $settings = [];
 
         // configuration
         $settings['default_view']     = (string) $this->rc->config->get('calendar_default_view', $this->defaults['calendar_default_view']);
         $settings['timeslots']        = (int) $this->rc->config->get('calendar_timeslots', $this->defaults['calendar_timeslots']);
         $settings['first_day']        = (int) $this->rc->config->get('calendar_first_day', $this->defaults['calendar_first_day']);
         $settings['first_hour']       = (int) $this->rc->config->get('calendar_first_hour', $this->defaults['calendar_first_hour']);
         $settings['work_start']       = (int) $this->rc->config->get('calendar_work_start', $this->defaults['calendar_work_start']);
         $settings['work_end']         = (int) $this->rc->config->get('calendar_work_end', $this->defaults['calendar_work_end']);
         $settings['agenda_range']     = (int) $this->rc->config->get('calendar_agenda_range', $this->defaults['calendar_agenda_range']);
         $settings['event_coloring']   = (int) $this->rc->config->get('calendar_event_coloring', $this->defaults['calendar_event_coloring']);
         $settings['time_indicator']   = (int) $this->rc->config->get('calendar_time_indicator', $this->defaults['calendar_time_indicator']);
         $settings['invite_shared']    = (int) $this->rc->config->get('calendar_allow_invite_shared', $this->defaults['calendar_allow_invite_shared']);
         $settings['itip_notify']      = (int) $this->rc->config->get('calendar_itip_send_option', $this->defaults['calendar_itip_send_option']);
         $settings['show_weekno']      = (int) $this->rc->config->get('calendar_show_weekno', $this->defaults['calendar_show_weekno']);
         $settings['default_calendar'] = $this->rc->config->get('calendar_default_calendar');
         $settings['invitation_calendars'] = (bool) $this->rc->config->get('kolab_invitation_calendars', false);
 
         // 'table' view has been replaced by 'list' view
         if ($settings['default_view'] == 'table') {
             $settings['default_view'] = 'list';
         }
 
         // get user identity to create default attendee
         if ($this->ui->screen == 'calendar') {
             foreach ($this->rc->user->list_emails() as $rec) {
                 if (empty($identity)) {
                     $identity = $rec;
                 }
 
                 $identity['emails'][] = $rec['email'];
                 $settings['identities'][$rec['identity_id']] = $rec['email'];
             }
 
             $identity['emails'][] = $this->rc->user->get_username();
             $identity['ownedResources'] = $this->owned_resources_emails();
             $settings['identity'] = [
                 'name'   => $identity['name'],
                 'email'  => strtolower($identity['email']),
                 'emails' => ';' . strtolower(join(';', $identity['emails'])),
                 'ownedResources' => ';' . strtolower(join(';', $identity['ownedResources']))
             ];
         }
 
         // freebusy token authentication URL
         if (($url = $this->rc->config->get('calendar_freebusy_session_auth_url'))
             && ($uniqueid = $this->rc->config->get('kolab_uniqueid'))
         ) {
             if ($url === true) {
                 $url = '/freebusy';
             }
             $url = rtrim(rcube_utils::resolve_url($url), '/ ');
             $url .= '/' . urlencode($this->rc->get_user_name());
             $url .= '/' . urlencode($uniqueid);
 
             $settings['freebusy_url'] = $url;
         }
 
         return $settings;
     }
 
     /**
      * Encode events as JSON
      *
      * @param array Events as array
      * @param bool  Add CSS class names according to calendar and categories
      *
      * @return string JSON encoded events
      */
     function encode($events, $addcss = false)
     {
         $json = [];
         foreach ($events as $event) {
             $json[] = $this->_client_event($event, $addcss);
         }
         return rcube_output::json_serialize($json);
     }
 
     /**
      * Convert an event object to be used on the client
      */
     private function _client_event($event, $addcss = false)
     {
         // compose a human readable strings for alarms_text and recurrence_text
         if (!empty($event['valarms'])) {
             $event['alarms_text'] = libcalendaring::alarms_text($event['valarms']);
             $event['valarms'] = libcalendaring::to_client_alarms($event['valarms']);
         }
 
         if (!empty($event['recurrence'])) {
             $event['recurrence_text'] = $this->lib->recurrence_text($event['recurrence']);
             $event['recurrence'] = $this->lib->to_client_recurrence($event['recurrence'], $event['allday']);
             unset($event['recurrence_date']);
         }
 
         if (!empty($event['attachments'])) {
             foreach ($event['attachments'] as $k => $attachment) {
                 $event['attachments'][$k]['classname'] = rcube_utils::file2class($attachment['mimetype'], $attachment['name']);
 
                 unset($event['attachments'][$k]['data'], $event['attachments'][$k]['content']);
 
                 if (empty($attachment['id'])) {
                     $event['attachments'][$k]['id'] = $k;
                 }
             }
         }
 
         // convert link URIs references into structs
         if (array_key_exists('links', $event)) {
             foreach ((array) $event['links'] as $i => $link) {
                 if (strpos($link, 'imap://') === 0 && ($msgref = $this->driver->get_message_reference($link))) {
                     $event['links'][$i] = $msgref;
                 }
             }
         }
 
         // check for organizer in attendees list
         $organizer = null;
         foreach ((array) $event['attendees'] as $i => $attendee) {
             if ($attendee['role'] == 'ORGANIZER') {
                 $organizer = $attendee;
             }
             if (!empty($attendee['status']) && $attendee['status'] == 'DELEGATED' && empty($attendee['rsvp'])) {
                 $event['attendees'][$i]['noreply'] = true;
             }
             else {
                 unset($event['attendees'][$i]['noreply']);
             }
         }
 
         if ($organizer === null && !empty($event['organizer'])) {
             $organizer = $event['organizer'];
             $organizer['role'] = 'ORGANIZER';
             if (!is_array($event['attendees'])) {
                 $event['attendees'] = [$organizer];
             }
         }
 
         // Convert HTML description into plain text
         if ($this->is_html($event)) {
             $h2t = new rcube_html2text($event['description'], false, true, 0);
             $event['description'] = trim($h2t->get_text());
         }
 
         // mapping url => vurl, allday => allDay because of the fullcalendar client script
         $event['vurl']   = $event['url'] ?? null;
         $event['allDay'] = !empty($event['allday']);
         unset($event['url']);
         unset($event['allday']);
 
         $event['className'] = !empty($event['className']) ? explode(' ', $event['className']) : [];
 
         if ($event['allDay']) {
             $event['end'] = $event['end']->add(new DateInterval('P1D'));
         }
 
         if (!empty($_GET['mode']) && $_GET['mode'] == 'print') {
             $event['editable'] = false;
         }
 
         return [
             '_id'     => $event['calendar'] . ':' . $event['id'],  // unique identifier for fullcalendar
             'start'   => $this->lib->adjust_timezone($event['start'], $event['allDay'])->format('c'),
             'end'     => $this->lib->adjust_timezone($event['end'], $event['allDay'])->format('c'),
             // 'changed' might be empty for event recurrences (Bug #2185)
             'changed' => !empty($event['changed']) ? $this->lib->adjust_timezone($event['changed'])->format('c') : null,
             'created' => !empty($event['created']) ? $this->lib->adjust_timezone($event['created'])->format('c') : null,
             'title'       => strval($event['title'] ?? null),
             'description' => strval($event['description'] ?? null),
             'location'    => strval($event['location'] ?? null),
         ] + $event;
     }
 
     /**
      * Generate a unique identifier for an event
      */
     public function generate_uid()
     {
         return strtoupper(md5(time() . uniqid(rand())) . '-' . substr(md5($this->rc->user->get_username()), 0, 16));
     }
 
     /**
      * TEMPORARY: generate random event data for testing
      * Create events by opening http://<roundcubeurl>/?_task=calendar&_action=randomdata&_num=500&_date=2014-08-01&_dev=120
      */
     public function generate_randomdata()
     {
         @set_time_limit(0);
 
         $num   = !empty($_REQUEST['_num']) ? intval($_REQUEST['_num']) : 100;
         $date  = !empty($_REQUEST['_date']) ? $_REQUEST['_date'] : 'now';
         $dev   = !empty($_REQUEST['_dev']) ? $_REQUEST['_dev'] : 30;
         $cats  = array_keys($this->driver->list_categories());
         $cals  = $this->driver->list_calendars(calendar_driver::FILTER_ACTIVE);
         $count = 0;
 
         while ($count++ < $num) {
             $spread   = intval($dev) * 86400; // days
             $refdate  = strtotime($date);
             $start    = round(($refdate + rand(-$spread, $spread)) / 600) * 600;
             $duration = round(rand(30, 360) / 30) * 30 * 60;
             $allday   = rand(0,20) > 18;
             $alarm    = rand(-30,12) * 5;
             $fb       = rand(0,2);
 
             if (date('G', $start) > 23) {
                 $start -= 3600;
             }
 
             if ($allday) {
                 $start    = strtotime(date('Y-m-d 00:00:00', $start));
                 $duration = 86399;
             }
 
             $title = '';
             $len = rand(2, 12);
             $words = explode(" ", "The Hough transform is named after Paul Hough who patented the method in 1962."
                 . " It is a technique which can be used to isolate features of a particular shape within an image."
                 . " Because it requires that the desired features be specified in some parametric form, the classical"
                 . " Hough transform is most commonly used for the de- tection of regular curves such as lines, circles,"
                 . " ellipses, etc. A generalized Hough transform can be employed in applications where a simple"
                 . " analytic description of a feature(s) is not possible. Due to the computational complexity of"
                 . " the generalized Hough algorithm, we restrict the main focus of this discussion to the classical"
                 . " Hough transform. Despite its domain restrictions, the classical Hough transform (hereafter"
                 . " referred to without the classical prefix ) retains many applications, as most manufac- tured"
                 . " parts (and many anatomical parts investigated in medical imagery) contain feature boundaries"
                 . " which can be described by regular curves. The main advantage of the Hough transform technique"
                 . " is that it is tolerant of gaps in feature boundary descriptions and is relatively unaffected"
                 . " by image noise.");
             // $chars = "!# abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ 1234567890";
             for ($i = 0; $i < $len; $i++) {
                 $title .= $words[rand(0,count($words)-1)] . " ";
             }
 
             $this->driver->new_event([
                 'uid'        => $this->generate_uid(),
                 'start'      => new DateTime('@'.$start),
                 'end'        => new DateTime('@'.($start + $duration)),
                 'allday'     => $allday,
                 'title'      => rtrim($title),
                 'free_busy'  => $fb == 2 ? 'outofoffice' : ($fb ? 'busy' : 'free'),
                 'categories' => $cats[array_rand($cats)],
                 'calendar'   => array_rand($cals),
                 'alarms'     => $alarm > 0 ? "-{$alarm}M:DISPLAY" : '',
                 'priority'   => rand(0,9),
             ]);
         }
 
         $this->rc->output->redirect('');
     }
 
     /**
      * Handler for attachments upload
      */
     public function attachment_upload()
     {
         $handler = new kolab_attachments_handler();
         $handler->attachment_upload(self::SESSION_KEY, 'cal-');
     }
 
     /**
      * Handler for attachments download/displaying
      */
     public function attachment_get()
     {
         $handler = new kolab_attachments_handler();
 
         // show loading page
         if (!empty($_GET['_preload'])) {
             return $handler->attachment_loading_page();
         }
 
         $event_id = rcube_utils::get_input_value('_event', rcube_utils::INPUT_GPC);
         $calendar = rcube_utils::get_input_value('_cal', rcube_utils::INPUT_GPC);
         $id       = rcube_utils::get_input_value('_id', rcube_utils::INPUT_GPC);
         $rev      = rcube_utils::get_input_value('_rev', rcube_utils::INPUT_GPC);
 
         $event = ['id' => $event_id, 'calendar' => $calendar, 'rev' => $rev];
 
         if ($calendar == '--invitation--itip') {
             $uid  = rcube_utils::get_input_value('_uid', rcube_utils::INPUT_GPC);
             $part = rcube_utils::get_input_value('_part', rcube_utils::INPUT_GPC);
             $mbox = rcube_utils::get_input_value('_mbox', rcube_utils::INPUT_GPC);
 
             $event      = $this->lib->mail_get_itip_object($mbox, $uid, $part, 'event');
             $attachment = $event['attachments'][$id];
             $attachment['body'] = &$attachment['data'];
         }
         else {
             $attachment = $this->driver->get_attachment($id, $event);
         }
 
         // show part page
         if (!empty($_GET['_frame'])) {
             $handler->attachment_page($attachment);
         }
         // deliver attachment content
         else if ($attachment) {
             if ($calendar != '--invitation--itip') {
                 $attachment['body'] = $this->driver->get_attachment_body($id, $event);
             }
 
             $handler->attachment_get($attachment);
         }
 
         // if we arrive here, the requested part was not found
         header('HTTP/1.1 404 Not Found');
         exit;
     }
 
     /**
      * Determine whether the given event description is HTML formatted
      */
     private function is_html($event)
     {
           // check for opening and closing <html> or <body> tags
         return !empty($event['description'])
             && preg_match('/<(html|body)(\s+[a-z]|>)/', $event['description'], $m)
             && strpos($event['description'], '</'.$m[1].'>') > 0;
     }
 
     /**
      * Prepares new/edited event properties before save
      */
     private function write_preprocess(&$event, $action)
     {
         // Remove double timezone specification (T2313)
         $event['start'] = preg_replace('/\s*\(.*\)/', '', $event['start']);
         $event['end']   = preg_replace('/\s*\(.*\)/', '', $event['end']);
 
         // convert dates into DateTime objects in user's current timezone
         $event['start']  = new DateTime($event['start'], $this->timezone);
         $event['end']    = new DateTime($event['end'], $this->timezone);
         $event['allday'] = !empty($event['allDay']);
         unset($event['allDay']);
 
         // start/end is all we need for 'move' action (#1480)
         if ($action == 'move') {
             return true;
         }
 
         // convert the submitted recurrence settings
         if (!empty($event['recurrence'])) {
             $event['recurrence'] = $this->lib->from_client_recurrence($event['recurrence'], $event['start']);
 
             // align start date with the first occurrence
             if (!empty($event['recurrence']) && !empty($event['syncstart'])
                 && (empty($event['_savemode']) || $event['_savemode'] == 'all')
             ) {
                 $next = $this->find_first_occurrence($event);
 
                 if (!$next) {
                     $this->rc->output->show_message('calendar.recurrenceerror', 'error');
                     return false;
                 }
                 else if ($event['start'] != $next) {
                     $diff = $event['start']->diff($event['end'], true);
 
                     $event['start'] = $next;
                     $event['end']   = clone $next;
                     $event['end']->add($diff);
                 }
             }
         }
 
         // convert the submitted alarm values
         if (!empty($event['valarms'])) {
             $event['valarms'] = libcalendaring::from_client_alarms($event['valarms']);
         }
 
         $attachments = [];
         $eventid     = 'cal-' . (!empty($event['id']) ? $event['id'] : 'new');
 
         if (!empty($_SESSION[self::SESSION_KEY]) && $_SESSION[self::SESSION_KEY]['id'] == $eventid) {
             if (!empty($_SESSION[self::SESSION_KEY]['attachments'])) {
                 foreach ($_SESSION[self::SESSION_KEY]['attachments'] as $id => $attachment) {
                     if (!empty($event['attachments']) && in_array($id, $event['attachments'])) {
                         $attachments[$id] = $this->rc->plugins->exec_hook('attachment_get', $attachment);
                     }
                 }
             }
         }
 
         $event['attachments'] = $attachments;
 
         // convert link references into simple URIs
         if (array_key_exists('links', $event)) {
             $event['links'] = array_map(function($link) {
                     return is_array($link) ? $link['uri'] : strval($link);
                 },
                 (array) $event['links']
             );
         }
 
         // check for organizer in attendees
         if ($action == 'new' || $action == 'edit') {
             if (empty($event['attendees'])) {
                 $event['attendees'] = [];
             }
 
             $emails = $this->get_user_emails();
             $organizer = $owner = false;
 
             foreach ((array) $event['attendees'] as $i => $attendee) {
                 if ($attendee['role'] == 'ORGANIZER') {
                     $organizer = $i;
                 }
                 if (!empty($attendee['email']) && in_array(strtolower($attendee['email']), $emails)) {
                     $owner = $i;
                 }
                 if (!isset($attendee['rsvp'])) {
                     $event['attendees'][$i]['rsvp'] = true;
                 }
                 else if (is_string($attendee['rsvp'])) {
                     $event['attendees'][$i]['rsvp'] = $attendee['rsvp'] == 'true' || $attendee['rsvp'] == '1';
                 }
             }
 
             if (!empty($event['_identity'])) {
                 $identity = $this->rc->user->get_identity($event['_identity']);
             }
 
             // set new organizer identity
             if ($organizer !== false && !empty($identity)) {
                 $event['attendees'][$organizer]['name']  = $identity['name'];
                 $event['attendees'][$organizer]['email'] = $identity['email'];
             }
             // set owner as organizer if yet missing
             else if ($organizer === false && $owner !== false) {
                 $event['attendees'][$owner]['role'] = 'ORGANIZER';
                 unset($event['attendees'][$owner]['rsvp']);
             }
             // fallback to the selected identity
             else if ($organizer === false && !empty($identity)) {
                 $event['attendees'][] = [
                     'role'  => 'ORGANIZER',
                     'name'  => $identity['name'],
                     'email' => $identity['email'],
                 ];
             }
         }
 
         // mapping url => vurl because of the fullcalendar client script
         if (array_key_exists('vurl', $event)) {
             $event['url'] = $event['vurl'];
             unset($event['vurl']);
         }
 
         return true;
     }
 
     /**
      * Releases some resources after successful event save
      */
     private function cleanup_event(&$event)
     {
         // remove temp. attachment files
         if (!empty($_SESSION[self::SESSION_KEY]) && ($eventid = $_SESSION[self::SESSION_KEY]['id'])) {
             $this->rc->plugins->exec_hook('attachments_cleanup', ['group' => $eventid]);
             $this->rc->session->remove(self::SESSION_KEY);
         }
     }
 
     /**
      * Send out an invitation/notification to all event attendees
      */
     private function notify_attendees($event, $old, $action = 'edit', $comment = null, $rsvp = null)
     {
-        $is_cancelled = false;
-        if ($action == 'remove' || ($event['status'] == 'CANCELLED' && ($old['status'] ?? '') != $event['status'])) {
-            $event['cancelled'] = true;
-            $is_cancelled = true;
-        }
+        $is_cancelled = $action == 'remove'
+            || (!empty($event['status']) && $event['status'] == 'CANCELLED' && ($old['status'] ?? '') != $event['status']);
+
+        $event['cancelled'] = $is_cancelled;
 
         if ($rsvp === null) {
             $rsvp = !$old || ($event['sequence'] ?? 0) > ($old['sequence'] ?? 0);
         }
 
         $itip        = $this->load_itip();
         $emails      = $this->get_user_emails();
         $itip_notify = (int) $this->rc->config->get('calendar_itip_send_option', $this->defaults['calendar_itip_send_option']);
 
         // add comment to the iTip attachment
         $event['comment'] = $comment;
 
         // set a valid recurrence-id if this is a recurrence instance
         libcalendaring::identify_recurrence_instance($event);
 
         // compose multipart message using PEAR:Mail_Mime
         $method  = $action == 'remove' ? 'CANCEL' : 'REQUEST';
         $message = $itip->compose_itip_message($event, $method, $rsvp);
 
         // list existing attendees from $old event
         $old_attendees = [];
         if (!empty($old['attendees'])) {
             foreach ((array) $old['attendees'] as $attendee) {
                 $old_attendees[] = $attendee['email'];
             }
         }
 
         // send to every attendee
         $sent    = 0;
         $current = [];
         foreach ((array) $event['attendees'] as $attendee) {
             // skip myself for obvious reasons
             if (empty($attendee['email']) || in_array(strtolower($attendee['email']), $emails)) {
                 continue;
             }
 
             $current[] = strtolower($attendee['email']);
 
             // skip if notification is disabled for this attendee
             if (!empty($attendee['noreply']) && $itip_notify & 2) {
                 continue;
             }
 
             // skip if this attendee has delegated and set RSVP=FALSE
             if ($attendee['status'] == 'DELEGATED' && $attendee['rsvp'] === false) {
                 continue;
             }
 
             // which template to use for mail text
             $is_new   = !in_array($attendee['email'], $old_attendees);
             $is_rsvp  = $is_new || $event['sequence'] > $old['sequence'];
             $bodytext = $is_cancelled ? 'eventcancelmailbody' : ($is_new ? 'invitationmailbody' : 'eventupdatemailbody');
             $subject  = $is_cancelled ? 'eventcancelsubject'  : ($is_new ? 'invitationsubject' : ($event['title'] ? 'eventupdatesubject' : 'eventupdatesubjectempty'));
 
             $event['comment'] = $comment;
 
             // finally send the message
             if ($itip->send_itip_message($event, $method, $attendee, $subject, $bodytext, $message, $is_rsvp)) {
                 $sent++;
             }
             else {
                 $sent = -100;
             }
         }
 
         // TODO: on change of a recurring (main) event, also send updates to differing attendess of recurrence exceptions
 
         // send CANCEL message to removed attendees
         if (!empty($old['attendees'])) {
             foreach ($old['attendees'] as $attendee) {
                 if ($attendee['role'] == 'ORGANIZER'
                     || empty($attendee['email'])
                     || in_array(strtolower($attendee['email']), $current)
                 ) {
                     continue;
                 }
 
                 $vevent = $old;
                 $vevent['cancelled'] = $is_cancelled;
                 $vevent['attendees'] = [$attendee];
                 $vevent['comment']   = $comment;
 
                 if ($itip->send_itip_message($vevent, 'CANCEL', $attendee, 'eventcancelsubject', 'eventcancelmailbody')) {
                     $sent++;
                 }
                 else {
                     $sent = -100;
                 }
             }
         }
 
         return $sent;
     }
 
     /**
      * Echo simple free/busy status text for the given user and time range
      */
     public function freebusy_status()
     {
         $email = rcube_utils::get_input_value('email', rcube_utils::INPUT_GPC);
         $start = $this->input_timestamp('start', rcube_utils::INPUT_GPC);
         $end   = $this->input_timestamp('end', rcube_utils::INPUT_GPC);
 
         if (!$start) $start = time();
         if (!$end) $end = $start + 3600;
 
         $status = 'UNKNOWN';
         $fbtypemap = [
             calendar::FREEBUSY_UNKNOWN   => 'UNKNOWN',
             calendar::FREEBUSY_FREE      => 'FREE',
             calendar::FREEBUSY_BUSY      => 'BUSY',
             calendar::FREEBUSY_TENTATIVE => 'TENTATIVE',
             calendar::FREEBUSY_OOF       => 'OUT-OF-OFFICE'
         ];
 
         // if the backend has free-busy information
         $fblist = $this->driver->get_freebusy_list($email, $start, $end);
 
         if (is_array($fblist)) {
             $status = 'FREE';
 
             foreach ($fblist as $slot) {
                 list($from, $to, $type) = $slot;
                 if ($from < $end && $to > $start) {
                     $status = isset($type) && !empty($fbtypemap[$type]) ? $fbtypemap[$type] : 'BUSY';
                     break;
                 }
             }
         }
 
         // let this information be cached for 5min
         $this->rc->output->future_expire_header(300);
 
         echo $status;
         exit;
     }
 
     /**
      * Return a list of free/busy time slots within the given period
      * Echo data in JSON encoding
      */
     public function freebusy_times()
     {
         $email = rcube_utils::get_input_value('email', rcube_utils::INPUT_GPC);
         $start = $this->input_timestamp('start', rcube_utils::INPUT_GPC);
         $end   = $this->input_timestamp('end', rcube_utils::INPUT_GPC);
         $interval  = intval(rcube_utils::get_input_value('interval', rcube_utils::INPUT_GPC));
         $strformat = $interval > 60 ? 'Ymd' : 'YmdHis';
 
         if (!$start) $start = time();
         if (!$end)   $end = $start + 86400 * 30;
         if (!$interval) $interval = 60;  // 1 hour
 
         if (!$dte) {
             $dts = new DateTime('@'.$start);
             $dts->setTimezone($this->timezone);
         }
 
         $fblist = $this->driver->get_freebusy_list($email, $start, $end);
         $slots  = '';
 
         // prepare freebusy list before use (for better performance)
         if (is_array($fblist)) {
             foreach ($fblist as $idx => $slot) {
                 list($from, $to, ) = $slot;
 
                 // check for possible all-day times
                 if (gmdate('His', $from) == '000000' && gmdate('His', $to) == '235959') {
                     // shift into the user's timezone for sane matching
                     $fblist[$idx][0] -= $this->gmt_offset;
                     $fblist[$idx][1] -= $this->gmt_offset;
                 }
             }
         }
 
         // build a list from $start till $end with blocks representing the fb-status
         for ($s = 0, $t = $start; $t <= $end; $s++) {
             $t_end = $t + $interval * 60;
             $dt = new DateTime('@'.$t);
             $dt->setTimezone($this->timezone);
 
             // determine attendee's status
             if (is_array($fblist)) {
                 $status = self::FREEBUSY_FREE;
 
                 foreach ($fblist as $slot) {
                     list($from, $to, $type) = $slot;
 
                     if ($from < $t_end && $to > $t) {
                         $status = isset($type) ? $type : self::FREEBUSY_BUSY;
                         if ($status == self::FREEBUSY_BUSY) {
                             // can't get any worse :-)
                             break;
                         }
                     }
                 }
             }
             else {
                 $status = self::FREEBUSY_UNKNOWN;
             }
 
             // use most compact format, assume $status is one digit/character
             $slots .= $status;
             $t = $t_end;
         }
 
         $dte = new DateTime('@'.$t_end);
         $dte->setTimezone($this->timezone);
 
         // let this information be cached for 5min
         $this->rc->output->future_expire_header(300);
 
         echo rcube_output::json_serialize([
             'email' => $email,
             'start' => $dts->format('c'),
             'end'   => $dte->format('c'),
             'interval' => $interval,
             'slots' => $slots,
         ]);
         exit;
     }
 
     /**
      * Handler for printing calendars
      */
     public function print_view()
     {
         $title = $this->gettext('print');
 
         $view = rcube_utils::get_input_value('view', rcube_utils::INPUT_GPC);
         if (!in_array($view, ['agendaWeek', 'agendaDay', 'month', 'list'])) {
             $view = 'agendaDay';
         }
 
         $this->rc->output->set_env('view', $view);
 
         if ($date = rcube_utils::get_input_value('date', rcube_utils::INPUT_GPC)) {
             $this->rc->output->set_env('date', $date);
         }
 
         if ($range = rcube_utils::get_input_value('range', rcube_utils::INPUT_GPC)) {
             $this->rc->output->set_env('listRange', intval($range));
         }
 
         if ($search = rcube_utils::get_input_value('search', rcube_utils::INPUT_GPC)) {
             $this->rc->output->set_env('search', $search);
             $title .= ' "' . $search . '"';
         }
 
         // Add JS to the page
         $this->ui->addJS();
 
         $this->register_handler('plugin.calendar_css', [$this->ui, 'calendar_css']);
         $this->register_handler('plugin.calendar_list', [$this->ui, 'calendar_list']);
 
         $this->rc->output->set_pagetitle($title);
         $this->rc->output->send('calendar.print');
     }
 
     /**
      * Compare two event objects and return differing properties
      *
      * @param array Event A
      * @param array Event B
      *
      * @return array List of differing event properties
      */
     public static function event_diff($a, $b)
     {
         $diff   = [];
         $ignore = ['changed' => 1, 'attachments' => 1];
 
         foreach (array_unique(array_merge(array_keys($a), array_keys($b))) as $key) {
             if (empty($ignore[$key]) && $key[0] != '_') {
                 $av = isset($a[$key]) ? $a[$key] : null;
                 $bv = isset($b[$key]) ? $b[$key] : null;
 
                 if ($av != $bv) {
                     $diff[] = $key;
                 }
             }
         }
 
         // only compare number of attachments
         $ac = !empty($a['attachments']) ? count($a['attachments']) : 0;
         $bc = !empty($b['attachments']) ? count($b['attachments']) : 0;
 
         if ($ac != $bc) {
             $diff[] = 'attachments';
         }
 
         return $diff;
     }
 
     /**
      * Update attendee properties on the given event object
      *
      * @param array The event object to be altered
      * @param array List of hash arrays each represeting an updated/added attendee
      */
     public static function merge_attendee_data(&$event, $attendees, $removed = null)
     {
         if (!empty($attendees) && !is_array($attendees[0])) {
             $attendees = [$attendees];
         }
 
         foreach ($attendees as $attendee) {
             $found = false;
 
             foreach ($event['attendees'] as $i => $candidate) {
                 if ($candidate['email'] == $attendee['email']) {
                     $event['attendees'][$i] = $attendee;
                     $found = true;
                     break;
                 }
             }
 
             if (!$found) {
                 $event['attendees'][] = $attendee;
             }
         }
 
         // filter out removed attendees
         if (!empty($removed)) {
             $event['attendees'] = array_filter($event['attendees'], function($attendee) use ($removed) {
                 return !in_array($attendee['email'], $removed);
             });
         }
     }
 
     /****  Resource management functions  ****/
 
     /**
      * Getter for the configured implementation of the resource directory interface
      */
     private function resources_directory()
     {
         if (!empty($this->resources_dir)) {
             return $this->resources_dir;
         }
 
         if ($driver_name = $this->rc->config->get('calendar_resources_driver')) {
             $driver_class = 'resources_driver_' . $driver_name;
 
             require_once($this->home . '/drivers/resources_driver.php');
             require_once($this->home . '/drivers/' . $driver_name . '/' . $driver_class . '.php');
 
             $this->resources_dir = new $driver_class($this);
         }
 
         return $this->resources_dir;
     }
 
     /**
      * Handler for resoruce autocompletion requests
      */
     public function resources_autocomplete()
     {
         $search  = rcube_utils::get_input_value('_search', rcube_utils::INPUT_GPC, true);
         $sid     = rcube_utils::get_input_value('_reqid', rcube_utils::INPUT_GPC);
         $maxnum  = (int)$this->rc->config->get('autocomplete_max', 15);
         $results = [];
 
         if ($directory = $this->resources_directory()) {
             foreach ($directory->load_resources($search, $maxnum) as $rec) {
                 $results[]  = [
                     'name'  => $rec['name'],
                     'email' => $rec['email'],
                     'type'  => $rec['_type'],
                 ];
             }
         }
 
         $this->rc->output->command('ksearch_query_results', $results, $search, $sid);
         $this->rc->output->send();
     }
 
     /**
      * Handler for load-requests for resource data
      */
     function resources_list()
     {
         $data = [];
 
         if ($directory = $this->resources_directory()) {
             foreach ($directory->load_resources() as $rec) {
                 $data[] = $rec;
             }
         }
 
         $this->rc->output->command('plugin.resource_data', $data);
         $this->rc->output->send();
     }
 
     /**
      * Handler for requests loading resource owner information
      */
     function resources_owner()
     {
         if ($directory = $this->resources_directory()) {
             $id   = rcube_utils::get_input_value('_id', rcube_utils::INPUT_GPC);
             $data = $directory->get_resource_owner($id);
         }
 
         $this->rc->output->command('plugin.resource_owner', $data);
         $this->rc->output->send();
     }
 
     /**
      * Deliver event data for a resource's calendar
      */
     function resources_calendar()
     {
         $events = [];
 
         if ($directory = $this->resources_directory()) {
             $id    = rcube_utils::get_input_value('_id', rcube_utils::INPUT_GPC);
             $start = $this->input_timestamp('start', rcube_utils::INPUT_GET);
             $end   = $this->input_timestamp('end', rcube_utils::INPUT_GET);
 
             $events = $directory->get_resource_calendar($id, $start, $end);
         }
 
         echo $this->encode($events);
         exit;
     }
 
     /**
      * List email addressed of owned resources
      */
     private function owned_resources_emails()
     {
         $results = [];
         if ($directory = $this->resources_directory()) {
             foreach ($directory->load_resources($_SESSION['kolab_dn'], 5000, 'owner') as $rec) {
                 $results[] = $rec['email'];
             }
         }
         return $results;
     }
 
 
     /****  Event invitation plugin hooks ****/
 
     /**
      * Find an event in user calendars
      */
     protected function find_event($event, &$mode)
     {
         $this->load_driver();
 
         // We search for writeable calendars in personal namespace by default
         $mode   = calendar_driver::FILTER_WRITEABLE | calendar_driver::FILTER_PERSONAL;
         $result = $this->driver->get_event($event, $mode);
         // ... now check shared folders if not found
         if (!$result) {
             $result = $this->driver->get_event($event, calendar_driver::FILTER_WRITEABLE | calendar_driver::FILTER_SHARED);
             if ($result) {
                 $mode |= calendar_driver::FILTER_SHARED;
             }
         }
 
         return $result;
     }
 
     /**
      * Handler for calendar/itip-status requests
      */
     function event_itip_status()
     {
         $data = rcube_utils::get_input_value('data', rcube_utils::INPUT_POST, true);
 
         $this->load_driver();
 
         // find local copy of the referenced event (in personal namespace)
         $existing  = $this->find_event($data, $mode);
         $is_shared = $mode & calendar_driver::FILTER_SHARED;
         $itip      = $this->load_itip();
         $response  = $itip->get_itip_status($data, $existing);
 
         // get a list of writeable calendars to save new events to
         if (
             (!$existing || $is_shared)
             && empty($data['nosave'])
             && ($response['action'] == 'rsvp' || $response['action'] == 'import')
         ) {
             $calendars       = $this->driver->list_calendars($mode);
             $calendar_select = new html_select([
                 'name'       => 'calendar',
                 'id'         => 'itip-saveto',
                 'is_escaped' => true,
                 'class'      => 'form-control custom-select'
             ]);
 
             $calendar_select->add('--', '');
             $numcals = 0;
             foreach ($calendars as $calendar) {
                 if (!empty($calendar['editable'])) {
                     $calendar_select->add($calendar['name'], $calendar['id']);
                     $numcals++;
                 }
             }
             if ($numcals < 1) {
                 $calendar_select = null;
             }
         }
 
         if (!empty($calendar_select)) {
             $default_calendar   = $this->get_default_calendar($calendars);
             $response['select'] = html::span('folder-select', $this->gettext('saveincalendar')
                 . '&nbsp;'
                 . $calendar_select->show($is_shared ? $existing['calendar'] : $default_calendar['id'])
             );
         }
         else if (!empty($data['nosave'])) {
             $response['select'] = html::tag('input', ['type' => 'hidden', 'name' => 'calendar', 'id' => 'itip-saveto', 'value' => '']);
         }
 
         // render small agenda view for the respective day
         if ($data['method'] == 'REQUEST' && !empty($data['date']) && $response['action'] == 'rsvp') {
             $event_start = rcube_utils::anytodatetime($data['date']);
             $day_start   = new Datetime(gmdate('Y-m-d 00:00', $data['date']), $this->lib->timezone);
             $day_end     = new Datetime(gmdate('Y-m-d 23:59', $data['date']), $this->lib->timezone);
 
             // get events on that day from the user's personal calendars
             $calendars = $this->driver->list_calendars(calendar_driver::FILTER_PERSONAL);
             $events    = $this->driver->load_events($day_start->format('U'), $day_end->format('U'), null, array_keys($calendars));
 
             usort($events, function($a, $b) { return $a['start'] > $b['start'] ? 1 : -1; });
 
             $before = $after = [];
             foreach ($events as $event) {
                 // TODO: skip events with free_busy == 'free' ?
                 if ($event['uid'] == $data['uid']
                     || $event['end'] < $day_start || $event['start'] > $day_end
                     || $event['status'] == 'CANCELLED'
                     || (!empty($event['className']) && strpos($event['className'], 'declined') !== false)
                 ) {
                     continue;
                 }
 
                 if ($event['start'] < $event_start) {
                     $before[] = $this->mail_agenda_event_row($event);
                 }
                 else {
                     $after[] = $this->mail_agenda_event_row($event);
                 }
             }
 
             $response['append'] = [
                 'selector' => '.calendar-agenda-preview',
                 'replacements' => [
                     '%before%' => !empty($before) ? join("\n", array_slice($before,  -3)) : html::div('event-row no-event', $this->gettext('noearlierevents')),
                     '%after%'  => !empty($after)  ? join("\n", array_slice($after, 0, 3)) : html::div('event-row no-event', $this->gettext('nolaterevents')),
                 ],
             ];
         }
 
         $this->rc->output->command('plugin.update_itip_object_status', $response);
     }
 
     /**
      * Handler for calendar/itip-remove requests
      */
     function event_itip_remove()
     {
         $uid      = rcube_utils::get_input_value('uid', rcube_utils::INPUT_POST);
         $instance = rcube_utils::get_input_value('_instance', rcube_utils::INPUT_POST);
         $savemode = rcube_utils::get_input_value('_savemode', rcube_utils::INPUT_POST);
         $listmode = calendar_driver::FILTER_WRITEABLE | calendar_driver::FILTER_PERSONAL;
         $success  = false;
 
         // search for event if only UID is given
         if ($event = $this->driver->get_event(['uid' => $uid, '_instance' => $instance], $listmode)) {
             $event['_savemode'] = $savemode;
             $success = $this->driver->remove_event($event, true);
         }
 
         if ($success) {
             $this->rc->output->show_message('calendar.successremoval', 'confirmation');
         }
         else {
             $this->rc->output->show_message('calendar.errorsaving', 'error');
         }
     }
 
     /**
      * Handler for URLs that allow an invitee to respond on his invitation mail
      */
     public function itip_attend_response($p)
     {
         $this->setup();
 
         if ($p['action'] == 'attend') {
             $this->ui->init();
 
             $this->rc->output->set_env('task', 'calendar');  // override some env vars
             $this->rc->output->set_env('refresh_interval', 0);
             $this->rc->output->set_pagetitle($this->gettext('calendar'));
 
             $itip  = $this->load_itip();
             $token = rcube_utils::get_input_value('_t', rcube_utils::INPUT_GPC);
 
             // read event info stored under the given token
             if ($invitation = $itip->get_invitation($token)) {
                 $this->token = $token;
                 $this->event = $invitation['event'];
 
                 // show message about cancellation
                 if (!empty($invitation['cancelled'])) {
                     $this->invitestatus = html::div('rsvp-status declined', $itip->gettext('eventcancelled'));
                 }
                 // save submitted RSVP status
                 else if (!empty($_POST['rsvp'])) {
                     $status = null;
                     foreach (['accepted', 'tentative', 'declined'] as $method) {
                         if ($_POST['rsvp'] == $itip->gettext('itip' . $method)) {
                             $status = $method;
                             break;
                         }
                     }
 
                     // send itip reply to organizer
                     $invitation['event']['comment'] = rcube_utils::get_input_value('_comment', rcube_utils::INPUT_POST);
                     if ($status && $itip->update_invitation($invitation, $invitation['attendee'], strtoupper($status))) {
                         $this->invitestatus = html::div('rsvp-status ' . strtolower($status), $itip->gettext('youhave'.strtolower($status)));
                     }
                     else {
                         $this->rc->output->command('display_message', $this->gettext('errorsaving'), 'error', -1);
                     }
 
                     // if user is logged in...
                     // FIXME: we should really consider removing this functionality
                     //        it's confusing that it creates/updates an event only for logged-in user
                     //        what if the logged-in user is not the same as the attendee?
                     if ($this->rc->user->ID) {
                         $this->load_driver();
 
                         $invitation = $itip->get_invitation($token);
                         $existing   = $this->driver->get_event($this->event);
 
                         // save the event to his/her default calendar if not yet present
                         if (!$existing && ($calendar = $this->get_default_calendar())) {
                             $invitation['event']['calendar'] = $calendar['id'];
                             if ($this->driver->new_event($invitation['event'])) {
                                 $msg = $this->gettext(['name' => 'importedsuccessfully', 'vars' => ['calendar' => $calendar['name']]]);
                                 $this->rc->output->command('display_message', $msg, 'confirmation');
                             }
                             else {
                                 $this->rc->output->command('display_message', $this->gettext('errorimportingevent'), 'error');
                             }
                         }
                         else if ($existing
                             && ($this->event['sequence'] >= $existing['sequence']
                                 || $this->event['changed'] >= $existing['changed'])
                             && ($calendar = $this->driver->get_calendar($existing['calendar']))
                         ) {
                             $this->event       = $invitation['event'];
                             $this->event['id'] = $existing['id'];
 
                             unset($this->event['comment']);
 
                             // merge attendees status
                             // e.g. preserve my participant status for regular updates
                             $this->lib->merge_attendees($this->event, $existing, $status);
 
                             // update attachments list
                             $event['deleted_attachments'] = true;
 
                             // show me as free when declined (#1670)
                             if ($status == 'declined') {
                                 $this->event['free_busy'] = 'free';
                             }
 
                             if ($this->driver->edit_event($this->event)) {
                                 $msg = $this->gettext(['name' => 'updatedsuccessfully', 'vars' => ['calendar' => $calendar->get_name()]]);
                                 $this->rc->output->command('display_message', $msg, 'confirmation');
                             }
                             else {
                                 $this->rc->output->command('display_message', $this->gettext('errorimportingevent'), 'error');
                             }
                         }
                     }
                 }
 
                 $this->register_handler('plugin.event_inviteform', [$this, 'itip_event_inviteform']);
                 $this->register_handler('plugin.event_invitebox', [$this->ui, 'event_invitebox']);
 
                 if (empty($this->invitestatus)) {
                     $this->itip->set_rsvp_actions(['accepted', 'tentative', 'declined']);
                     $this->register_handler('plugin.event_rsvp_buttons', [$this->ui, 'event_rsvp_buttons']);
                 }
 
                 $this->rc->output->set_pagetitle($itip->gettext('itipinvitation') . ' ' . $this->event['title']);
             }
             else {
                 $this->rc->output->command('display_message', $this->gettext('itipinvalidrequest'), 'error', -1);
             }
 
             $this->rc->output->send('calendar.itipattend');
         }
     }
 
     /**
      *
      */
     public function itip_event_inviteform($attrib)
     {
         $hidden = new html_hiddenfield(['name' => "_t", 'value' => $this->token]);
 
         return html::tag('form', [
                 'action' => $this->rc->url(['task' => 'calendar', 'action' => 'attend']),
                 'method' => 'post',
                 'noclose' => true
             ] + $attrib
         ) . $hidden->show();
     }
 
     /**
      *
      */
     private function mail_agenda_event_row($event, $class = '')
     {
         if (!empty($event['allday'])) {
             $time = $this->gettext('all-day');
         }
         else {
             $start = is_object($event['start']) ? clone $event['start'] : $event['start'];
             $end = is_object($event['end']) ? clone $event['end'] : $event['end'];
 
             $time = $this->rc->format_date($start, $this->rc->config->get('time_format'))
                 . ' - ' . $this->rc->format_date($end, $this->rc->config->get('time_format'));
         }
 
         return html::div(rtrim('event-row ' . ($class ?: $event['className'])),
             html::span('event-date', $time)
             . html::span('event-title', rcube::Q($event['title']))
         );
     }
 
     /**
      *
      */
     public function mail_messages_list($p)
     {
         if (!empty($p['cols']) && in_array('attachment', (array) $p['cols']) && !empty($p['messages'])) {
             foreach ($p['messages'] as $header) {
                 $part = new StdClass;
                 $part->mimetype = $header->ctype;
 
                 if (libcalendaring::part_is_vcalendar($part)) {
                     $header->list_flags['attachmentClass'] = 'ical';
                 }
                 else if (in_array($header->ctype, ['multipart/alternative', 'multipart/mixed'])) {
                     // TODO: fetch bodystructure and search for ical parts. Maybe too expensive?
                     if (!empty($header->structure) && !empty($header->structure->parts)) {
                         foreach ($header->structure->parts as $part) {
                             if (libcalendaring::part_is_vcalendar($part)
                                 && !empty($part->ctype_parameters['method'])
                             ) {
                                 $header->list_flags['attachmentClass'] = 'ical';
                                 break;
                             }
                         }
                     }
                 }
             }
         }
     }
 
     /**
      * Add UI element to copy event invitations or updates to the calendar
      */
     public function mail_messagebody_html($p)
     {
         // load iCalendar functions (if necessary)
         if (!empty($this->lib->ical_parts)) {
             $this->get_ical();
             $this->load_itip();
         }
 
         $html = '';
         $has_events = false;
         $ical_objects = $this->lib->get_mail_ical_objects();
 
         // show a box for every event in the file
         foreach ($ical_objects as $idx => $event) {
             if ($event['_type'] != 'event') {
                 // skip non-event objects (#2928)
                 continue;
             }
 
             $has_events = true;
 
             // get prepared inline UI for this event object
             if ($ical_objects->method) {
                 $append   = '';
                 $date_str = $this->rc->format_date(clone $event['start'], $this->rc->config->get('date_format'), empty($event['start']->_dateonly));
                 $date     = new DateTime($event['start']->format('Y-m-d') . ' 12:00:00', new DateTimeZone('UTC'));
 
                 // prepare a small agenda preview to be filled with actual event data on async request
                 if ($ical_objects->method == 'REQUEST') {
                     $append = html::div('calendar-agenda-preview',
                         html::tag('h3', 'preview-title', $this->gettext('agenda') . ' ' . html::span('date', $date_str))
                         . '%before%' . $this->mail_agenda_event_row($event, 'current') . '%after%'
                     );
                 }
 
                 $html .= html::div('calendar-invitebox invitebox boxinformation',
                     $this->itip->mail_itip_inline_ui(
                         $event,
                         $ical_objects->method,
                         $ical_objects->mime_id . ':' . $idx,
                         'calendar',
                         rcube_utils::anytodatetime($ical_objects->message_date),
                         $this->rc->url(['task' => 'calendar']) . '&view=agendaDay&date=' . $date->format('U')
                     ) . $append
                 );
             }
 
             // limit listing
             if ($idx >= 3) {
                 break;
             }
         }
 
         // prepend event boxes to message body
         if ($html) {
             $this->ui->init();
             $p['content'] = $html . $p['content'];
             $this->rc->output->add_label('calendar.savingdata','calendar.deleteventconfirm','calendar.declinedeleteconfirm');
         }
 
         // add "Save to calendar" button into attachment menu
         if ($has_events) {
             $this->add_button([
                     'id'         => 'attachmentsavecal',
                     'name'       => 'attachmentsavecal',
                     'type'       => 'link',
                     'wrapper'    => 'li',
                     'command'    => 'attachment-save-calendar',
                     'class'      => 'icon calendarlink disabled',
                     'classact'   => 'icon calendarlink active',
                     'innerclass' => 'icon calendar',
                     'label'      => 'calendar.savetocalendar',
                 ],
                 'attachmentmenu'
             );
         }
 
         return $p;
     }
 
     /**
      * Handler for POST request to import an event attached to a mail message
      */
     public function mail_import_itip()
     {
         $itip_sending = $this->rc->config->get('calendar_itip_send_option', $this->defaults['calendar_itip_send_option']);
 
         $uid     = rcube_utils::get_input_value('_uid', rcube_utils::INPUT_POST);
         $mbox    = rcube_utils::get_input_value('_mbox', rcube_utils::INPUT_POST);
         $mime_id = rcube_utils::get_input_value('_part', rcube_utils::INPUT_POST);
         $status  = rcube_utils::get_input_value('_status', rcube_utils::INPUT_POST);
         $delete  = intval(rcube_utils::get_input_value('_del', rcube_utils::INPUT_POST));
         $noreply = intval(rcube_utils::get_input_value('_noreply', rcube_utils::INPUT_POST));
         $noreply = $noreply || $status == 'needs-action' || $itip_sending === 0;
         $instance = rcube_utils::get_input_value('_instance', rcube_utils::INPUT_POST);
         $savemode = rcube_utils::get_input_value('_savemode', rcube_utils::INPUT_POST);
         $comment  = rcube_utils::get_input_value('_comment', rcube_utils::INPUT_POST);
 
         $error_msg = $this->gettext('errorimportingevent');
         $success   = false;
         $deleted   = false;
 
         if ($status == 'delegated') {
             $to = rcube_utils::get_input_value('_to', rcube_utils::INPUT_POST, true);
             $delegates = rcube_mime::decode_address_list($to, 1, false);
             $delegate  = reset($delegates);
 
             if (empty($delegate) || empty($delegate['mailto'])) {
                 $this->rc->output->command('display_message', $this->rc->gettext('libcalendaring.delegateinvalidaddress'), 'error');
                 return;
             }
         }
 
         // successfully parsed events?
         if ($event = $this->lib->mail_get_itip_object($mbox, $uid, $mime_id, 'event')) {
             // forward iTip request to delegatee
             if (!empty($delegate)) {
                 $rsvpme = rcube_utils::get_input_value('_rsvp', rcube_utils::INPUT_POST);
                 $itip   = $this->load_itip();
 
                 $event['comment'] = $comment;
 
                 if ($itip->delegate_to($event, $delegate, !empty($rsvpme))) {
                     $this->rc->output->show_message('calendar.itipsendsuccess', 'confirmation');
                 }
                 else {
                     $this->rc->output->command('display_message', $this->gettext('itipresponseerror'), 'error');
                 }
 
                 unset($event['comment']);
 
                 // the delegator is set to non-participant, thus save as non-blocking
                 $event['free_busy'] = 'free';
             }
 
             $mode = calendar_driver::FILTER_PERSONAL
                 | calendar_driver::FILTER_SHARED
                 | calendar_driver::FILTER_WRITEABLE;
 
             // find writeable calendar to store event
             $cal_id    = rcube_utils::get_input_value('_folder', rcube_utils::INPUT_POST);
             $dontsave  = $cal_id === '' && $event['_method'] == 'REQUEST';
             $calendars = $this->driver->list_calendars($mode);
             $calendar  = isset($calendars[$cal_id]) ? $calendars[$cal_id] : null;
 
             // select default calendar except user explicitly selected 'none'
             if (!$calendar && !$dontsave) {
                 $calendar = $this->get_default_calendar($calendars);
             }
 
             $metadata = [
                 'uid'       => $event['uid'],
                 '_instance' => isset($event['_instance']) ? $event['_instance'] : null,
                 'changed'   => is_object($event['changed']) ? $event['changed']->format('U') : 0,
                 'sequence'  => intval($event['sequence']),
                 'fallback'  => strtoupper($status),
                 'method'    => $event['_method'],
                 'task'      => 'calendar',
             ];
 
             // update my attendee status according to submitted method
             if (!empty($status)) {
                 $organizer = null;
                 $emails = $this->get_user_emails();
                 foreach ($event['attendees'] as $i => $attendee) {
                     if ($attendee['role'] == 'ORGANIZER') {
                         $organizer = $attendee;
                     }
                     else if (!empty($attendee['email']) && in_array(strtolower($attendee['email']), $emails)) {
                         $event['attendees'][$i]['status'] = strtoupper($status);
                         if (!in_array($event['attendees'][$i]['status'], ['NEEDS-ACTION', 'DELEGATED'])) {
                             $event['attendees'][$i]['rsvp'] = false;  // unset RSVP attribute
                         }
 
                         $metadata['attendee'] = $attendee['email'];
                         $metadata['rsvp']     = $attendee['role'] != 'NON-PARTICIPANT';
 
                         $reply_sender   = $attendee['email'];
                         $event_attendee = $attendee;
                     }
                 }
 
                 // add attendee with this user's default identity if not listed
                 if (!$reply_sender) {
                     $sender_identity = $this->rc->user->list_emails(true);
                     $event['attendees'][] = [
                         'name'   => $sender_identity['name'],
                         'email'  => $sender_identity['email'],
                         'role'   => 'OPT-PARTICIPANT',
                         'status' => strtoupper($status),
                     ];
                     $metadata['attendee'] = $sender_identity['email'];
                 }
             }
 
             // save to calendar
             if ($calendar && !empty($calendar['editable'])) {
                 // check for existing event with the same UID
                 $existing = $this->find_event($event, $mode);
 
                 // we'll create a new copy if user decided to change the calendar
                 if ($existing && $cal_id && $calendar && $calendar['id'] != $existing['calendar']) {
                     $existing = null;
                 }
 
                 if ($existing) {
                     $calendar = $calendars[$existing['calendar']];
 
                     // forward savemode for correct updates of recurring events
                     $existing['_savemode'] = $savemode ?: (!empty($event['_savemode']) ? $event['_savemode'] : null);
 
                     // only update attendee status
                     if ($event['_method'] == 'REPLY') {
                         $existing_attendee_index  = -1;
 
                         $event_attendee   = null;
                         $update_attendees = [];
 
                         if ($attendee = $this->itip->find_reply_attendee($event)) {
                             $event_attendee       = $attendee;
                             $update_attendees[]   = $attendee;
                             $metadata['fallback'] = $attendee['status'];
                             $metadata['attendee'] = $attendee['email'];
                             $metadata['rsvp']     = !empty($attendee['rsvp']) || $attendee['role'] != 'NON-PARTICIPANT';
 
                             $existing_attendee_emails = [];
 
                             // Find the attendee to update
                             foreach ($existing['attendees'] as $i => $existing_attendee) {
                                 $existing_attendee_emails[] = $existing_attendee['email'];
                                 if ($this->itip->compare_email($existing_attendee['email'], $attendee['email'])) {
                                     $existing_attendee_index = $i;
                                 }
                             }
 
                             if ($attendee['status'] == 'DELEGATED') {
                                 //Also find and copy the delegatee
                                 $delegatee_email = $attendee['email'];
                                 $delegatees = array_filter($event['attendees'], function($attendee) use ($delegatee_email){ return $attendee['role'] != 'ORGANIZER' && $this->itip->compare_email($attendee['delegated-from'], $delegatee_email); });
 
                                 if ($delegatee = $this->itip->find_attendee_by_email($event['attendees'], 'delegated-from', $attendee['email'])) {
                                     $update_attendees[] = $delegatee;
                                     if (!in_array_nocase($delegatee['email'], $existing_attendee_emails)) {
                                         $existing['attendees'][] = $delegated_attendee;
                                     }
                                 }
                             }
                         }
 
                         // if delegatee has declined, set delegator's RSVP=True
                         if ($event_attendee
                             && $event_attendee['status'] == 'DECLINED'
                             && !empty($event_attendee['delegated-from'])
                         ) {
                             foreach ($existing['attendees'] as $i => $attendee) {
                                 if ($attendee['email'] == $event_attendee['delegated-from']) {
                                     $existing['attendees'][$i]['rsvp'] = true;
                                     break;
                                 }
                             }
                         }
 
                         // found matching attendee entry in both existing and new events
                         if ($existing_attendee_index >= 0 && $event_attendee) {
                             $existing['attendees'][$existing_attendee_index] = $event_attendee;
                             $success = $this->driver->update_attendees($existing, $update_attendees);
                         }
                         // update the entire attendees block
                         else if (
                             ($event['sequence'] >= $existing['sequence'] || $event['changed'] >= $existing['changed'])
                             && $event_attendee
                         ) {
                             $existing['attendees'][] = $event_attendee;
                             $success = $this->driver->update_attendees($existing, $update_attendees);
                         }
                         else if (!$event_attendee) {
                             $error_msg = $this->gettext('errorunknownattendee');
                         }
                         else {
                             $error_msg = $this->gettext('newerversionexists');
                         }
                     }
                     // delete the event when declined (#1670)
                     else if ($status == 'declined' && $delete) {
                         $deleted = $this->driver->remove_event($existing, true);
                         $success = true;
                     }
                     // import the (newer) event
                     else if ($event['sequence'] >= $existing['sequence'] || $event['changed'] >= $existing['changed']) {
                         $event['id']       = $existing['id'];
                         $event['calendar'] = $existing['calendar'];
 
                         // merge attendees status
                         // e.g. preserve my participant status for regular updates
                         $this->lib->merge_attendees($event, $existing, $status);
 
                         // set status=CANCELLED on CANCEL messages
                         if ($event['_method'] == 'CANCEL') {
                             $event['status'] = 'CANCELLED';
                         }
 
                         // update attachments list, allow attachments update only on REQUEST (#5342)
                         if ($event['_method'] == 'REQUEST') {
                             $event['deleted_attachments'] = true;
                         }
                         else {
                             unset($event['attachments']);
                         }
 
                         // show me as free when declined (#1670)
                         if ($status == 'declined'
                             || (!empty($event['status']) && $event['status'] == 'CANCELLED')
                             || $event_attendee['role'] == 'NON-PARTICIPANT'
                         ) {
                             $event['free_busy'] = 'free';
                         }
 
                         $success = $this->driver->edit_event($event);
                     }
                     else if (!empty($status)) {
                         $existing['attendees'] = $event['attendees'];
                         if ($status == 'declined' || $event_attendee['role'] == 'NON-PARTICIPANT') {
                             // show me as free when declined (#1670)
                             $existing['free_busy'] = 'free';
                         }
                         $success = $this->driver->edit_event($existing);
                     }
                     else {
                         $error_msg = $this->gettext('newerversionexists');
                     }
                 }
                 else if (!$existing && ($status != 'declined' || $this->rc->config->get('kolab_invitation_calendars'))) {
                     if ($status == 'declined'
                         || $event['status'] == 'CANCELLED'
                         || $event_attendee['role'] == 'NON-PARTICIPANT'
                     ) {
                         $event['free_busy'] = 'free';
                     }
 
                     // if the RSVP reply only refers to a single instance:
                     // store unmodified master event with current instance as exception
                     if (!empty($instance) && !empty($savemode) && $savemode != 'all') {
                         $master = $this->lib->mail_get_itip_object($mbox, $uid, $mime_id, 'event');
                         if ($master['recurrence'] && empty($master['_instance'])) {
                             // compute recurring events until this instance's date
                             if ($recurrence_date = rcube_utils::anytodatetime($instance, $master['start']->getTimezone())) {
                                 $recurrence_date->setTime(23,59,59);
 
                                 foreach ($this->driver->get_recurring_events($master, $master['start'], $recurrence_date) as $recurring) {
                                     if ($recurring['_instance'] == $instance) {
                                         // copy attendees block with my partstat to exception
                                         $recurring['attendees'] = $event['attendees'];
                                         $master['recurrence']['EXCEPTIONS'][] = $recurring;
                                         $event = $recurring;  // set reference for iTip reply
                                         break;
                                     }
                                 }
 
                                 $master['calendar'] = $event['calendar'] = $calendar['id'];
                                 $success = $this->driver->new_event($master);
                             }
                             else {
                                 $master = null;
                             }
                         }
                         else {
                             $master = null;
                         }
                     }
 
                     // save to the selected/default calendar
                     if (!$master) {
                         $event['calendar'] = $calendar['id'];
                         $success = $this->driver->new_event($event);
                     }
                 }
                 else if ($status == 'declined') {
                     $error_msg = null;
                 }
             }
             else if ($status == 'declined' || $dontsave) {
                 $error_msg = null;
             }
             else {
                 $error_msg = $this->gettext('nowritecalendarfound');
             }
         }
 
         if ($success) {
             if ($event['_method'] == 'REPLY') {
                 $message = 'attendeupdateesuccess';
             }
             else {
                 $message = $deleted ? 'successremoval' : ($existing ? 'updatedsuccessfully' : 'importedsuccessfully');
             }
 
             $msg = $this->gettext(['name' => $message, 'vars' => ['calendar' => $calendar['name']]]);
             $this->rc->output->command('display_message', $msg, 'confirmation');
         }
 
         if ($success || $dontsave) {
             $metadata['calendar'] = isset($event['calendar']) ? $event['calendar'] : null;
             $metadata['nosave']   = $dontsave;
             $metadata['rsvp']     = !empty($metadata['rsvp']);
 
             $metadata['after_action'] = $this->rc->config->get('calendar_itip_after_action', $this->defaults['calendar_itip_after_action']);
             $this->rc->output->command('plugin.itip_message_processed', $metadata);
             $error_msg = null;
         }
         else if ($error_msg) {
             $this->rc->output->command('display_message', $error_msg, 'error');
         }
 
         // send iTip reply
         if ($event['_method'] == 'REQUEST' && !empty($organizer) && !$noreply
             && !in_array(strtolower($organizer['email']), $emails) && !$error_msg
         ) {
             $event['comment'] = $comment;
             $itip = $this->load_itip();
             $itip->set_sender_email($reply_sender);
 
             if ($itip->send_itip_message($event, 'REPLY', $organizer, 'itipsubject' . $status, 'itipmailbody' . $status)) {
                 $mailto = $organizer['name'] ? $organizer['name'] : $organizer['email'];
                 $msg    = $this->gettext(['name' => 'sentresponseto', 'vars' => ['mailto' => $mailto]]);
                 $this->rc->output->command('display_message', $msg, 'confirmation');
             }
             else {
                 $this->rc->output->command('display_message', $this->gettext('itipresponseerror'), 'error');
             }
         }
 
         $this->rc->output->send();
     }
 
     /**
      * Handler for calendar/itip-remove requests
      */
     function mail_itip_decline_reply()
     {
         $uid     = rcube_utils::get_input_value('_uid', rcube_utils::INPUT_POST);
         $mbox    = rcube_utils::get_input_value('_mbox', rcube_utils::INPUT_POST);
         $mime_id = rcube_utils::get_input_value('_part', rcube_utils::INPUT_POST);
 
         if (($event = $this->lib->mail_get_itip_object($mbox, $uid, $mime_id, 'event'))
             && $event['_method'] == 'REPLY'
         ) {
             $event['comment'] = rcube_utils::get_input_value('_comment', rcube_utils::INPUT_POST);
 
             foreach ($event['attendees'] as $_attendee) {
                 if ($_attendee['role'] != 'ORGANIZER') {
                     $attendee = $_attendee;
                     break;
                 }
             }
 
             $itip = $this->load_itip();
 
             if ($itip->send_itip_message($event, 'CANCEL', $attendee, 'itipsubjectcancel', 'itipmailbodycancel')) {
                 $mailto = !empty($attendee['name']) ? $attendee['name'] : $attendee['email'];
                 $msg    = $this->gettext(['name' => 'sentresponseto', 'vars' => ['mailto' => $mailto]]);
                 $this->rc->output->command('display_message', $msg, 'confirmation');
             }
             else {
                 $this->rc->output->command('display_message', $this->gettext('itipresponseerror'), 'error');
             }
         }
         else {
             $this->rc->output->command('display_message', $this->gettext('itipresponseerror'), 'error');
         }
     }
 
     /**
      * Handler for calendar/itip-delegate requests
      */
     function mail_itip_delegate()
     {
         // forward request to mail_import_itip() with the right status
         $_POST['_status'] = $_REQUEST['_status'] = 'delegated';
         $this->mail_import_itip();
     }
 
     /**
      * Import the full payload from a mail message attachment
      */
     public function mail_import_attachment()
     {
         $uid     = rcube_utils::get_input_value('_uid', rcube_utils::INPUT_POST);
         $mbox    = rcube_utils::get_input_value('_mbox', rcube_utils::INPUT_POST);
         $mime_id = rcube_utils::get_input_value('_part', rcube_utils::INPUT_POST);
         $charset = RCUBE_CHARSET;
 
         // establish imap connection
         $imap = $this->rc->get_storage();
         $imap->set_folder($mbox);
 
         if ($uid && $mime_id) {
             $part = $imap->get_message_part($uid, $mime_id);
             // $headers = $imap->get_message_headers($uid);
 
             if ($part) {
                 if (!empty($part->ctype_parameters['charset'])) {
                     $charset = $part->ctype_parameters['charset'];
                 }
                 $events = $this->get_ical()->import($part, $charset);
             }
         }
 
         $success = $existing = 0;
 
         if (!empty($events)) {
             // find writeable calendar to store event
             $cal_id = !empty($_REQUEST['_calendar']) ? rcube_utils::get_input_value('_calendar', rcube_utils::INPUT_POST) : null;
             $calendars = $this->driver->list_calendars(calendar_driver::FILTER_PERSONAL);
 
             foreach ($events as $event) {
                 // save to calendar
                 $calendar = !empty($calendars[$cal_id]) ? $calendars[$cal_id] : $this->get_default_calendar();
                 if ($calendar && $calendar['editable'] && $event['_type'] == 'event') {
                     $event['calendar'] = $calendar['id'];
 
                     if (!$this->driver->get_event($event['uid'], calendar_driver::FILTER_WRITEABLE)) {
                         $success += (bool)$this->driver->new_event($event);
                     }
                     else {
                         $existing++;
                     }
                 }
             }
         }
 
         if ($success) {
             $msg = $this->gettext(['name' => 'importsuccess', 'vars' => ['nr' => $success]]);
             $this->rc->output->command('display_message', $msg, 'confirmation');
         }
         else if ($existing) {
             $this->rc->output->command('display_message', $this->gettext('importwarningexists'), 'warning');
         }
         else {
             $this->rc->output->command('display_message', $this->gettext('errorimportingevent'), 'error');
         }
     }
 
     /**
      * Read email message and return contents for a new event based on that message
      */
     public function mail_message2event()
     {
         $this->ui->init();
         $this->ui->addJS();
         $this->ui->init_templates();
         $this->ui->calendar_list([], true); // set env['calendars']
 
         $uid   = rcube_utils::get_input_value('_uid', rcube_utils::INPUT_GET);
         $mbox  = rcube_utils::get_input_value('_mbox', rcube_utils::INPUT_GET);
         $event = [];
 
         // establish imap connection
         $imap    = $this->rc->get_storage();
         $message = new rcube_message($uid, $mbox);
 
         if ($message->headers) {
             $event['title']       = trim($message->subject);
             $event['description'] = trim($message->first_text_part());
 
             $this->load_driver();
 
             // add a reference to the email message
             if ($msgref = $this->driver->get_message_reference($message->headers, $mbox)) {
                 $event['links'] = [$msgref];
             }
             // copy mail attachments to event
             else if ($message->attachments) {
                 $eventid = 'cal-';
                 if (empty($_SESSION[self::SESSION_KEY]) || $_SESSION[self::SESSION_KEY]['id'] != $eventid) {
                     $_SESSION[self::SESSION_KEY] = [
                         'id'          => $eventid,
                         'attachments' => [],
                     ];
                 }
 
                 foreach ((array) $message->attachments as $part) {
                     $attachment = [
                         'data' => $imap->get_message_part($uid, $part->mime_id, $part),
                         'size' => $part->size,
                         'name' => $part->filename,
                         'mimetype' => $part->mimetype,
                         'group' => $eventid,
                     ];
 
                     $attachment = $this->rc->plugins->exec_hook('attachment_save', $attachment);
 
                     if (!empty($attachment['status']) && !$attachment['abort']) {
                         $id = $attachment['id'];
                         $attachment['classname'] = rcube_utils::file2class($attachment['mimetype'], $attachment['name']);
 
                         // store new attachment in session
                         unset($attachment['status'], $attachment['abort'], $attachment['data']);
                         $_SESSION[self::SESSION_KEY]['attachments'][$id] = $attachment;
 
                         $attachment['id'] = 'rcmfile' . $attachment['id'];  // add prefix to consider it 'new'
                         $event['attachments'][] = $attachment;
                     }
                 }
             }
 
             $this->rc->output->set_env('event_prop', $event);
         }
         else {
             $this->rc->output->command('display_message', $this->gettext('messageopenerror'), 'error');
         }
 
         $this->rc->output->send('calendar.dialog');
     }
 
     /**
      * Handler for the 'message_compose' plugin hook. This will check for
      * a compose parameter 'calendar_event' and create an attachment with the
      * referenced event in iCal format
      */
     public function mail_message_compose($args)
     {
         // set the submitted event ID as attachment
         if (!empty($args['param']['calendar_event'])) {
             $this->load_driver();
 
             list($cal, $id) = explode(':', $args['param']['calendar_event'], 2);
 
             if ($event = $this->driver->get_event(['id' => $id, 'calendar' => $cal])) {
                 $filename = asciiwords($event['title']);
                 if (empty($filename)) {
                     $filename = 'event';
                 }
 
                 // save ics to a temp file and register as attachment
                 $tmp_path = tempnam($this->rc->config->get('temp_dir'), 'rcmAttmntCal');
                 $export   = $this->get_ical()->export([$event], '', false, [$this->driver, 'get_attachment_body']);
 
                 file_put_contents($tmp_path, $export);
 
                 $args['attachments'][] = [
                     'path'     => $tmp_path,
                     'name'     => $filename . '.ics',
                     'mimetype' => 'text/calendar',
                     'size'     => filesize($tmp_path),
                 ];
                 $args['param']['subject'] = $event['title'];
             }
         }
 
         return $args;
     }
 
     /**
      * Create a Nextcould Talk room
      */
     public function talk_room_create()
     {
         require_once __DIR__ . '/lib/calendar_nextcloud_api.php';
 
         $api = new calendar_nextcloud_api();
 
         $name = (string) rcube_utils::get_input_value('_name', rcube_utils::INPUT_POST);
 
         $room_url = $api->talk_room_create($name);
 
         if ($room_url) {
             $this->rc->output->command('plugin.talk_room_created', ['url' => $room_url]);
         }
         else {
             $this->rc->output->command('display_message', $this->gettext('talkroomcreateerror'), 'error');
         }
     }
 
     /**
      * Update a Nextcould Talk room
      */
     public function talk_room_update($event)
     {
         // If a room is assigned to the event...
         if (
             ($talk_url = $this->rc->config->get('calendar_nextcloud_url'))
             && isset($event['attendees'])
             && !empty($event['location'])
             && strpos($event['location'], unslashify($talk_url) . '/call/') === 0
         ) {
             $participants = [];
             $organizer = null;
 
             // ollect participants' and organizer's email addresses
             foreach ($event['attendees'] as $attendee) {
                 if (!empty($attendee['email'])) {
                     if ($attendee['role'] == 'ORGANIZER') {
                         $organizer = $attendee['email'];
                     }
                     else if ($attendee['cutype'] == 'INDIVIDUAL') {
                         $participants[] = $attendee['email'];
                     }
                 }
             }
 
             // If the event is owned by the current user update the room
             if ($organizer && in_array($organizer, $this->get_user_emails())) {
                 require_once __DIR__ . '/lib/calendar_nextcloud_api.php';
 
                 $api = new calendar_nextcloud_api();
 
                 $api->talk_room_update($event['location'], $participants);
             }
         }
     }
 
     /**
      * Get a list of email addresses of the current user (from login and identities)
      */
     public function get_user_emails()
     {
         return $this->lib->get_user_emails();
     }
 
     /**
      * Build an absolute URL with the given parameters
      */
     public function get_url($param = [])
     {
         $param += ['task' => 'calendar'];
         return $this->rc->url($param, true, true);
     }
 
     public function ical_feed_hash($source)
     {
         return base64_encode($this->rc->user->get_username() . ':' . $source);
     }
 
     /**
      * Handler for user_delete plugin hook
      */
     public function user_delete($args)
     {
          // delete itipinvitations entries related to this user
          $db = $this->rc->get_dbh();
          $table_itipinvitations = $db->table_name('itipinvitations', true);
 
          $db->query("DELETE FROM $table_itipinvitations WHERE `user_id` = ?", $args['user']->ID);
 
         $this->setup();
         $this->load_driver();
 
         return $this->driver->user_delete($args);
     }
 
     /**
      * Find first occurrence of a recurring event excluding start date
      *
      * @param array $event Event data (with 'start' and 'recurrence')
      *
      * @return DateTime Date of the first occurrence
      */
     public function find_first_occurrence($event)
     {
         // Make sure libkolab/libcalendaring plugins are loaded
         $this->load_driver();
 
         $driver_name = $this->rc->config->get('calendar_driver', 'database');
 
         // Use kolabcalendaring/kolabformat to compute recurring events only with the Kolab driver
         if ($driver_name == 'kolab' && class_exists('kolabformat') && class_exists('kolabcalendaring')
             && class_exists('kolab_date_recurrence')
         ) {
             $object = kolab_format::factory('event', 3.0);
             $object->set($event);
 
             $recurrence = new kolab_date_recurrence($object);
         }
         else {
             // fallback to libcalendaring recurrence implementation
             $recurrence = new libcalendaring_recurrence($this->lib, $event);
         }
 
         return $recurrence->first_occurrence();
     }
 
     /**
      * Get date-time input from UI and convert to unix timestamp
      */
     protected function input_timestamp($name, $type)
     {
         $ts = rcube_utils::get_input_value($name, $type);
 
         if ($ts && (!is_numeric($ts) || strpos($ts, 'T'))) {
             $ts = new DateTime($ts, $this->timezone);
             $ts = $ts->getTimestamp();
         }
 
         return $ts;
     }
 
     /**
      * Magic getter for public access to protected members
      */
     public function __get($name)
     {
         switch ($name) {
         case 'ical':
             return $this->get_ical();
 
         case 'itip':
             return $this->load_itip();
 
         case 'driver':
             $this->load_driver();
             return $this->driver;
         }
 
         return null;
     }
 }
diff --git a/plugins/calendar/drivers/caldav/caldav_calendar.php b/plugins/calendar/drivers/caldav/caldav_calendar.php
index 85b8bb5c..873e1f4e 100644
--- a/plugins/calendar/drivers/caldav/caldav_calendar.php
+++ b/plugins/calendar/drivers/caldav/caldav_calendar.php
@@ -1,882 +1,880 @@
 <?php
 
 /**
  * CalDAV calendar storage class
  *
  * @author Aleksander Machniak <machniak@apheleia-it.ch>
  *
  * Copyright (C) 2012-2022, Apheleia IT AG <contact@apheleia-it.ch>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
  * published by the Free Software Foundation, either version 3 of the
  * License, or (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU Affero General Public License for more details.
  *
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
 class caldav_calendar extends kolab_storage_dav_folder
 {
     public $ready         = false;
     public $rights        = 'lrs';
     public $editable      = false;
     public $attachments   = true;
     public $alarms        = false;
     public $history       = false;
     public $subscriptions = false;
     public $categories    = [];
     public $storage;
 
     public $type = 'event';
 
     protected $cal;
     protected $events = [];
     protected $search_fields = ['title', 'description', 'location', 'attendees', 'categories'];
 
     /**
      * Factory method to instantiate a caldav_calendar object
      *
      * @param string $id       Calendar ID (encoded IMAP folder name)
      * @param object $calendar Calendar plugin object
      *
      * @return caldav_calendar Self instance
      */
     public static function factory($id, $calendar)
     {
         return new caldav_calendar($id, $calendar);
     }
 
     /**
      * Default constructor
      */
     public function __construct($folder_or_id, $calendar)
     {
         if ($folder_or_id instanceof kolab_storage_dav_folder) {
             $this->storage = $folder_or_id;
         }
         else {
             // $this->storage = kolab_storage_dav::get_folder($folder_or_id, 'event');
         }
 
         $this->cal        = $calendar;
         $this->id         = $this->storage->id;
         $this->attributes = $this->storage->attributes;
         $this->ready      = true;
         $this->alarms     = !isset($this->storage->attributes['alarms']) || $this->storage->attributes['alarms'];
 
         // Set writeable and alarms flags according to folder permissions
         if ($this->ready) {
             if ($this->storage->get_namespace() == 'personal') {
                 $this->editable = true;
                 $this->rights = 'lrswikxteav';
             }
             else {
                 $rights = $this->storage->get_myrights();
                 if ($rights) {
                     $this->rights = $rights;
                     if (strpos($rights, 't') !== false || strpos($rights, 'd') !== false) {
                         $this->editable = strpos($rights, 'i');;
                     }
                 }
             }
         }
 
         $this->default = $this->storage->default;
         $this->subtype = $this->storage->subtype;
     }
 
     /**
      * Getter for the folder name
      *
      * @return string Name of the folder
      */
     public function get_realname()
     {
         return $this->get_name();
     }
 
     /**
      * Return color to display this calendar
      */
     public function get_color($default = null)
     {
         if ($color = $this->storage->get_color()) {
             return $color;
         }
 
         return $default ?: 'cc0000';
     }
 
     /**
      * Compose an URL for CalDAV access to this calendar (if configured)
      */
     public function get_caldav_url()
     {
 /*
         if ($template = $this->cal->rc->config->get('calendar_caldav_url', null)) {
             return strtr($template, [
                     '%h' => $_SERVER['HTTP_HOST'],
                     '%u' => urlencode($this->cal->rc->get_user_name()),
                     '%i' => urlencode($this->storage->get_uid()),
                     '%n' => urlencode($this->name),
             ]);
         }
 */
         return false;
     }
 
     /**
      * Update properties of this calendar folder
      *
      * @see caldav_driver::edit_calendar()
      */
     public function update(&$prop)
     {
         return false; // NOP
     }
 
     /**
      * Getter for a single event object
      */
     public function get_event($id)
     {
         // remove our occurrence identifier if it's there
         $master_id = preg_replace('/-\d{8}(T\d{6})?$/', '', $id);
 
         // directly access storage object
         if (empty($this->events[$id]) && $master_id == $id && ($record = $this->storage->get_object($id))) {
             $this->events[$id] = $record = $this->_to_driver_event($record, true);
         }
 
         // maybe a recurring instance is requested
         if (empty($this->events[$id]) && $master_id != $id) {
             $instance_id = substr($id, strlen($master_id) + 1);
 
             if ($record = $this->storage->get_object($master_id)) {
                 $master = $record = $this->_to_driver_event($record);
             }
 
             if (!empty($master)) {
                 // check for match on the first instance already
                 if (!empty($master['_instance']) && $master['_instance'] == $instance_id) {
                     $this->events[$id] = $master;
                 }
                 else if (!empty($master['recurrence'])) {
                     $start_date = $master['start'];
                     // For performance reasons we'll get only the specific instance
                     if (($date = substr($id, strlen($master_id) + 1, 8)) && strlen($date) == 8 && is_numeric($date)) {
                         $start_date = new DateTime($date . 'T000000', $master['start']->getTimezone());
                     }
 
                     $this->get_recurring_events($record, $start_date, null, $id, 1);
                 }
             }
         }
 
         return $this->events[$id];
     }
 
     /**
      * Get attachment body
      * @see calendar_driver::get_attachment_body()
      */
     public function get_attachment_body($id, $event)
     {
         $event = $this->get_event($event['id']);
         $data = $this->storage->get_attachment($id, $event);
 /*
         if ($data === false) {
             // try again with master UID
             $uid = preg_replace('/-\d+(T\d{6})?$/', '', $event['id']);
             if ($uid != $event['id']) {
                 $data = $this->storage->get_attachment($uid, $id); // TODO
             }
         }
 */
         return $data;
     }
 
     /**
      * @param int    Event's new start (unix timestamp)
      * @param int    Event's new end (unix timestamp)
      * @param string Search query (optional)
      * @param bool   Include virtual events (optional)
      * @param array  Additional parameters to query storage
      * @param array  Additional query to filter events
      *
      * @return array A list of event records
      */
     public function list_events($start, $end, $search = null, $virtual = 1, $query = [], $filter_query = null)
     {
         // convert to DateTime for comparisons
         // #5190: make the range a little bit wider
         // to workaround possible timezone differences
         try {
             $start = new DateTime('@' . ($start - 12 * 3600));
         }
         catch (Exception $e) {
             $start = new DateTime('@0');
         }
         try {
             $end = new DateTime('@' . ($end + 12 * 3600));
         }
         catch (Exception $e) {
             $end = new DateTime('today +10 years');
         }
 
         // get email addresses of the current user
         $user_emails = $this->cal->get_user_emails();
 
         // query Kolab storage
         $query[] = ['dtstart', '<=', $end];
         $query[] = ['dtend',   '>=', $start];
 
         if (is_array($filter_query)) {
             $query = array_merge($query, $filter_query);
         }
 
         $words = [];
         $partstat_exclude = [];
         $events = [];
 
         if (!empty($search)) {
             $search = mb_strtolower($search);
             $words  = rcube_utils::tokenize_string($search, 1);
             foreach (rcube_utils::normalize_string($search, true) as $word) {
                 $query[] = ['words', 'LIKE', $word];
             }
         }
 
         // set partstat filter to skip pending and declined invitations
         if (empty($filter_query)
             && $this->cal->rc->config->get('kolab_invitation_calendars')
             && $this->get_namespace() != 'other'
         ) {
             $partstat_exclude = ['NEEDS-ACTION', 'DECLINED'];
         }
 
         foreach ($this->storage->select($query) as $record) {
             if (empty($record)) {
                 continue;
             }
 
             $event = $this->_to_driver_event($record, !$virtual, false);
 
             // remember seen categories
             if (!empty($event['categories'])) {
                 $cat = is_array($event['categories']) ? $event['categories'][0] : $event['categories'];
                 $this->categories[$cat]++;
             }
 
             // list events in requested time window
             if ($event['start'] <= $end && $event['end'] >= $start) {
                 unset($event['_attendees']);
                 $add = true;
 
                 // skip the first instance of a recurring event if listed in exdate
                 if ($virtual && !empty($event['recurrence']['EXDATE'])) {
                     $event_date = $event['start']->format('Ymd');
                     $event_tz   = $event['start']->getTimezone();
 
                     foreach ((array) $event['recurrence']['EXDATE'] as $exdate) {
                         $ex = clone $exdate;
                         $ex->setTimezone($event_tz);
 
                         if ($ex->format('Ymd') == $event_date) {
                             $add = false;
                             break;
                         }
                     }
                 }
 
                 // find and merge exception for the first instance
                 if ($virtual && !empty($event['recurrence']) && !empty($event['recurrence']['EXCEPTIONS'])) {
                     foreach ($event['recurrence']['EXCEPTIONS'] as $exception) {
-                        if ($event['_instance'] == $exception['_instance']) {
+                        if (libcalendaring::is_recurrence_exception($event, $exception)) {
                             unset($exception['calendar'], $exception['className'], $exception['_folder_id']);
                             // clone date objects from main event before adjusting them with exception data
                             if (is_object($event['start'])) {
                                 $event['start'] = clone $record['start'];
                             }
                             if (is_object($event['end'])) {
                                 $event['end'] = clone $record['end'];
                             }
+
                             kolab_driver::merge_exception_data($event, $exception);
                         }
                     }
                 }
 
                 if ($add) {
                     $events[] = $event;
                 }
             }
 
             // resolve recurring events
             if (!empty($event['recurrence']) && $virtual == 1) {
                 $events = array_merge($events, $this->get_recurring_events($event, $start, $end));
             }
             // add top-level exceptions (aka loose single occurrences)
             else if (!empty($record['exceptions'])) {
                 foreach ($record['exceptions'] as $ex) {
                     $component = $this->_to_driver_event($ex, false, false, $record);
                     if ($component['start'] <= $end && $component['end'] >= $start) {
                         $events[] = $component;
                     }
                 }
             }
         }
 
         // post-filter all events by fulltext search and partstat values
         $me = $this;
         $events = array_filter($events, function($event) use ($words, $partstat_exclude, $user_emails, $me) {
             // fulltext search
             if (count($words)) {
                 $hits = 0;
                 foreach ($words as $word) {
                     $hits += $me->fulltext_match($event, $word, false);
                 }
                 if ($hits < count($words)) {
                     return false;
                 }
             }
 
             // partstat filter
             if (count($partstat_exclude) && !empty($event['attendees'])) {
                 foreach ($event['attendees'] as $attendee) {
                     if (
                         in_array($attendee['email'], $user_emails)
                         && in_array($attendee['status'], $partstat_exclude)
                     ) {
                         return false;
                     }
                 }
             }
 
             return true;
         });
 
         // Apply event-to-mail relations
         // $config = kolab_storage_config::get_instance();
         // $config->apply_links($events);
 
-        // Avoid session race conditions that will loose temporary subscriptions
-        // $this->cal->rc->session->nowrite = true;
-
         return $events;
     }
 
     /**
      * Get number of events in the given calendar
      *
      * @param int   Date range start (unix timestamp)
      * @param int   Date range end (unix timestamp)
      * @param array Additional query to filter events
      *
      * @return int Number of events
      */
     public function count_events($start, $end = null, $filter_query = null)
     {
         // convert to DateTime for comparisons
         try {
             $start = new DateTime('@'.$start);
         }
         catch (Exception $e) {
             $start = new DateTime('@0');
         }
         if ($end) {
             try {
                 $end = new DateTime('@'.$end);
             }
             catch (Exception $e) {
                 $end = null;
             }
         }
 
         // query Kolab storage
         $query[] = ['dtend',   '>=', $start];
 
         if ($end) {
             $query[] = ['dtstart', '<=', $end];
         }
 
         // add query to exclude pending/declined invitations
         if (empty($filter_query)) {
             foreach ($this->cal->get_user_emails() as $email) {
                 $query[] = ['tags', '!=', 'x-partstat:' . $email . ':needs-action'];
                 $query[] = ['tags', '!=', 'x-partstat:' . $email . ':declined'];
             }
         }
         else if (is_array($filter_query)) {
             $query = array_merge($query, $filter_query);
         }
 
         return $this->storage->count($query);
     }
 
     /**
      * Create a new event record
      *
      * @see calendar_driver::new_event()
      *
      * @return array|false The created record ID on success, False on error
      */
     public function insert_event($event)
     {
         if (!is_array($event)) {
             return false;
         }
 
         // email links are stored separately
         // $links = !empty($event['links']) ? $event['links'] : [];
         // unset($event['links']);
 
         // generate new event from RC input
         $object = $this->_from_driver_event($event);
         $saved  = $this->storage->save($object, 'event');
 
         if (!$saved) {
             rcube::raise_error([
                     'code' => 600, 'file' => __FILE__, 'line' => __LINE__,
                     'message' => "Error saving event object to DAV server"
                 ],
                 true, false
             );
             return false;
         }
 
         // save links in configuration.relation object
         // if ($this->save_links($event['uid'], $links)) {
         //    $object['links'] = $links;
         // }
 
         $this->events = [$event['uid'] => $this->_to_driver_event($object, true)];
 
         return true;
     }
 
     /**
      * Update a specific event record
      *
      * @return bool True on success, False on error
      */
     public function update_event($event, $exception_id = null)
     {
         $updated = false;
         $old = $this->storage->get_object(!empty($event['uid']) ? $event['uid'] : $event['id']);
 
         if (!$old || PEAR::isError($old)) {
             return false;
         }
 
         // email links are stored separately
         // $links = !empty($event['links']) ? $event['links'] : [];
         // unset($event['links']);
 
         $object = $this->_from_driver_event($event, $old);
 
         $object['created'] = $old['created'] ?? null;
 
         $saved = $this->storage->save($object, 'event', $old['uid']);
 
         if (!$saved) {
             rcube::raise_error([
                     'code' => 600, 'file' => __FILE__, 'line' => __LINE__,
                     'message' => "Error saving event object to CalDAV server"
                 ],
                 true, false
             );
         }
         else {
             // save links in configuration.relation object
             // if ($this->save_links($event['uid'], $links)) {
             //     $object['links'] = $links;
             // }
 
             $updated = true;
             $this->events = [$event['uid'] => $this->_to_driver_event($object, true)];
 
             // refresh local cache with recurring instances
             if ($exception_id) {
                 $this->get_recurring_events($object, $event['start'], $event['end'], $exception_id);
             }
         }
 
         return $updated;
     }
 
     /**
      * Delete an event record
      *
      * @see calendar_driver::remove_event()
      *
      * @return bool True on success, False on error
      */
     public function delete_event($event, $force = true)
     {
         $uid = !empty($event['uid']) ? $event['uid'] : $event['id'];
         $deleted = $this->storage->delete($uid, $force);
 
         if (!$deleted) {
             rcube::raise_error([
                     'code' => 600, 'file' => __FILE__, 'line' => __LINE__,
                     'message' => "Error deleting event '{$uid}' from CalDAV server"
                 ],
                 true, false
             );
         }
 
         return $deleted;
     }
 
     /**
      * Restore deleted event record
      *
      * @see calendar_driver::undelete_event()
      *
      * @return bool True on success, False on error
      */
     public function restore_event($event)
     {
         // TODO
         return false;
     }
 
     /**
      * Find messages linked with an event
      */
     protected function get_links($uid)
     {
         return []; // TODO
         $storage = kolab_storage_config::get_instance();
         return $storage->get_object_links($uid);
     }
 
     /**
      * Save message references (links) to an event
      */
     protected function save_links($uid, $links)
     {
         return false; // TODO
         $storage = kolab_storage_config::get_instance();
         return $storage->save_object_links($uid, (array) $links);
     }
 
     /**
      * Create instances of a recurring event
      *
      * @param array    $event    Hash array with event properties
      * @param DateTime $start    Start date of the recurrence window
      * @param DateTime $end      End date of the recurrence window
      * @param string   $event_id ID of a specific recurring event instance
      * @param int      $limit    Max. number of instances to return
      *
      * @return array List of recurring event instances
      */
     public function get_recurring_events($event, $start, $end = null, $event_id = null, $limit = null)
     {
         // determine a reasonable end date if none given
         if (!$end) {
             $end = clone $event['start'];
             $end->add(new DateInterval('P100Y'));
         }
 
         // read recurrence exceptions first
         $events = [];
         $exdata = [];
         $futuredata = [];
         $recurrence_id_format = libcalendaring::recurrence_id_format($event);
 
         if (!empty($event['recurrence'])) {
             // copy the recurrence rule from the master event (to be used in the UI)
             $recurrence_rule = $event['recurrence'];
             unset($recurrence_rule['EXCEPTIONS'], $recurrence_rule['EXDATE']);
 
             if (!empty($event['recurrence']['EXCEPTIONS'])) {
                 foreach ($event['recurrence']['EXCEPTIONS'] as $exception) {
                     if (empty($exception['_instance'])) {
                         $exception['_instance'] = libcalendaring::recurrence_instance_identifier($exception, !empty($event['allday']));
                     }
 
                     $rec_event = $this->_to_driver_event($exception, false, false, $event);
                     $rec_event['id'] = $event['uid'] . '-' . $exception['_instance'];
                     $rec_event['isexception'] = 1;
 
                     // found the specifically requested instance: register exception (single occurrence wins)
                     if (
                         $rec_event['id'] == $event_id
                         && (empty($this->events[$event_id]) || !empty($this->events[$event_id]['thisandfuture']))
                     ) {
                         $rec_event['recurrence'] = $recurrence_rule;
                         $rec_event['recurrence_id'] = $event['uid'];
                         $this->events[$rec_event['id']] = $rec_event;
                     }
 
                     // remember this exception's date
                     $exdate = substr($exception['_instance'], 0, 8);
                     if (empty($exdata[$exdate]) || !empty($exdata[$exdate]['thisandfuture'])) {
                         $exdata[$exdate] = $rec_event;
                     }
                     if (!empty($rec_event['thisandfuture'])) {
                         $futuredata[$exdate] = $rec_event;
                     }
                 }
             }
         }
 
         // found the specifically requested instance, exiting...
         if ($event_id && !empty($this->events[$event_id])) {
             return [$this->events[$event_id]];
         }
 
         // Check first occurrence, it might have been moved
         if (!empty($exdata[$event['start']->format('Ymd')])) {
             $first = $exdata[$event['start']->format('Ymd')];
             // return it only if not already in the result, but in the requested period
             if (!($event['start'] <= $end && $event['end'] >= $start)
                 && ($first['start'] <= $end && $first['end'] >= $start)
             ) {
                 $events[] = $first;
             }
         }
 
         if ($limit && count($events) >= $limit) {
             return $events;
         }
 
-        // use libkolab to compute recurring events
+        // use libcalendaring to compute recurring events
         $recurrence = libcalendaring::get_recurrence($event);
 
         $i = 0;
         while ($next_event = $recurrence->next_instance()) {
             $datestr     = $next_event['start']->format('Ymd');
             $instance_id = $next_event['start']->format($recurrence_id_format);
 
             // use this event data for future recurring instances
             $overlay_data = $futuredata[$datestr] ?? null;
 
             $rec_id      = $event['uid'] . '-' . $instance_id;
             $exception   = !empty($exdata[$datestr]) ? $exdata[$datestr] : $overlay_data;
             $event_start = $next_event['start'];
             $event_end   = $next_event['end'];
 
             // copy some event from exception to get proper start/end dates
             if ($exception) {
                 $event_copy = $next_event;
                 caldav_driver::merge_exception_dates($event_copy, $exception);
                 $event_start = $event_copy['start'];
                 $event_end   = $event_copy['end'];
             }
 
             // add to output if in range
             if (($event_start <= $end && $event_end >= $start) || ($event_id && $rec_id == $event_id)) {
                 $rec_event = $this->_to_driver_event($next_event, false, false, $event);
                 $rec_event['_instance'] = $instance_id;
                 $rec_event['_count'] = $i + 1;
 
                 if ($exception) {
                     // copy data from exception
                     caldav_driver::merge_exception_data($rec_event, $exception);
                 }
 
                 $rec_event['id'] = $rec_id;
                 $rec_event['recurrence_id'] = $event['uid'];
                 $rec_event['recurrence'] = $recurrence_rule;
                 unset($rec_event['_attendees']);
                 $events[] = $rec_event;
 
                 if ($rec_id == $event_id) {
                     $this->events[$rec_id] = $rec_event;
                     break;
                 }
 
                 if ($limit && count($events) >= $limit) {
                     return $events;
                 }
             }
             else if ($next_event['start'] > $end) {
                 // stop loop if out of range
                 break;
             }
 
             // avoid endless recursion loops
             if (++$i > 100000) {
                 break;
             }
         }
 
         return $events;
     }
 
     /**
      * Convert from storage format to internal representation
      */
     private function _to_driver_event($record, $noinst = false, $links = true, $master_event = null)
     {
         $record['calendar'] = $this->id;
 
         // remove (possibly outdated) cached parameters
         unset($record['_folder_id'], $record['className']);
 
         // if ($links && !array_key_exists('links', $record)) {
         //    $record['links'] = $this->get_links($record['uid']);
         // }
 
         $ns = $this->get_namespace();
 
         if ($ns == 'other') {
             $record['className'] = 'fc-event-ns-other';
         }
 
         if ($ns == 'other' || !$this->cal->rc->config->get('kolab_invitation_calendars')) {
             $record = caldav_driver::add_partstat_class($record, ['NEEDS-ACTION', 'DECLINED'], $this->get_owner());
 
             // Modify invitation status class name, when invitation calendars are disabled
             // we'll use opacity only for declined/needs-action events
             $record['className'] = !empty($record['className']) ? str_replace('-invitation', '', $record['className']) : '';
         }
 
         // add instance identifier to first occurrence (master event)
         $recurrence_id_format = libcalendaring::recurrence_id_format($master_event ? $master_event : $record);
         if (!$noinst && !empty($record['recurrence']) && !empty($record['start'])
             && empty($record['recurrence_id']) && empty($record['_instance'])
         ) {
             $record['_instance'] = $record['start']->format($recurrence_id_format);
         }
         else if (isset($record['recurrence_date']) && $record['recurrence_date'] instanceof DateTimeInterface) {
             $record['_instance'] = $record['recurrence_date']->format($recurrence_id_format);
         }
 
         // clean up exception data
         if (!empty($record['recurrence']) && !empty($record['recurrence']['EXCEPTIONS'])) {
             array_walk($record['recurrence']['EXCEPTIONS'], function(&$exception) {
                 unset($exception['_attachments']);
             });
         }
 
         return $record;
     }
 
     /**
      * Convert the given event record into a data structure that can be passed to the storage backend for saving
      * (opposite of self::_to_driver_event())
      */
     private function _from_driver_event($event, $old = [])
     {
         // set current user as ORGANIZER
         if ($identity = $this->cal->rc->user->list_emails(true)) {
             $event['attendees'] = !empty($event['attendees']) ? $event['attendees'] : [];
             $found = false;
 
             // there can be only resources on attendees list (T1484)
             // let's check the existence of an organizer
             foreach ($event['attendees'] as $attendee) {
                 if (!empty($attendee['role']) && $attendee['role'] == 'ORGANIZER') {
                     $found = true;
                     break;
                 }
             }
 
             if (!$found) {
                 $event['attendees'][] = ['role' => 'ORGANIZER', 'name' => $identity['name'], 'email' => $identity['email']];
             }
 
             $event['_owner'] = $identity['email'];
         }
 
         // remove EXDATE values if RDATE is given
         if (!empty($event['recurrence']['RDATE'])) {
             $event['recurrence']['EXDATE'] = [];
         }
 
         // remove recurrence information (e.g. EXDATES and EXCEPTIONS) entirely
         if (!empty($event['recurrence']) && empty($event['recurrence']['FREQ']) && empty($event['recurrence']['RDATE'])) {
             $event['recurrence'] = [];
         }
 
         // keep 'comment' from initial itip invitation
         if (!empty($old['comment'])) {
             $event['comment'] = $old['comment'];
         }
 
         // remove some internal properties which should not be cached
         $cleanup_fn = function(&$event) {
             unset($event['_savemode'], $event['_fromcalendar'], $event['_identity'], $event['_folder_id'],
                 $event['calendar'], $event['className'], $event['recurrence_id'],
                 $event['attachments'], $event['deleted_attachments']);
         };
 
         $cleanup_fn($event);
 
         // clean up exception data
         if (!empty($event['exceptions'])) {
             array_walk($event['exceptions'], $cleanup_fn);
         }
 
         // copy meta data (starting with _) from old object
         foreach ((array) $old as $key => $val) {
             if (!isset($event[$key]) && $key[0] == '_') {
                 $event[$key] = $val;
             }
         }
 
         return $event;
     }
 
     /**
      * Match the given word in the event contents
      */
     public function fulltext_match($event, $word, $recursive = true)
     {
         $hits = 0;
         foreach ($this->search_fields as $col) {
             if (empty($event[$col])) {
                 continue;
             }
 
             $sval = is_array($event[$col]) ? self::_complex2string($event[$col]) : $event[$col];
             if (empty($sval)) {
                 continue;
             }
 
             // do a simple substring matching (to be improved)
             $val = mb_strtolower($sval);
             if (strpos($val, $word) !== false) {
                 $hits++;
                 break;
             }
         }
 
         return $hits;
     }
 
     /**
      * Convert a complex event attribute to a string value
      */
     private static function _complex2string($prop)
     {
         static $ignorekeys = ['role', 'status', 'rsvp'];
 
         $out = '';
         if (is_array($prop)) {
             foreach ($prop as $key => $val) {
                 if (is_numeric($key)) {
                     $out .= self::_complex2string($val);
                 }
                 else if (!in_array($key, $ignorekeys)) {
                     $out .= $val . ' ';
                 }
             }
         }
         else if (is_string($prop) || is_numeric($prop)) {
             $out .= $prop . ' ';
         }
 
         return rtrim($out);
     }
 }
diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index c326238a..84bfddb0 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -1,2659 +1,2666 @@
 <?php
 
 /**
  * Kolab driver for the Calendar plugin
  *
  * @version @package_version@
  * @author Thomas Bruederli <bruederli@kolabsys.com>
  * @author Aleksander Machniak <machniak@kolabsys.com>
  *
  * Copyright (C) 2012-2015, Kolab Systems AG <contact@kolabsys.com>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
  * published by the Free Software Foundation, either version 3 of the
  * License, or (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU Affero General Public License for more details.
  *
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
 class kolab_driver extends calendar_driver
 {
     const INVITATIONS_CALENDAR_PENDING  = '--invitation--pending';
     const INVITATIONS_CALENDAR_DECLINED = '--invitation--declined';
 
     // features this backend supports
     public $alarms              = true;
     public $attendees           = true;
     public $freebusy            = true;
     public $attachments         = true;
     public $undelete            = true;
     public $alarm_types         = ['DISPLAY', 'AUDIO'];
     public $categoriesimmutable = true;
 
     protected $rc;
     protected $cal;
     protected $calendars;
     protected $storage;
     protected $has_writeable    = false;
     protected $freebusy_trigger = false;
     protected $bonnie_api       = false;
 
     /**
      * Default constructor
      */
     public function __construct($cal)
     {
         $cal->require_plugin('libkolab');
 
         // load helper classes *after* libkolab has been loaded (#3248)
         require_once(__DIR__ . '/kolab_calendar.php');
         require_once(__DIR__ . '/kolab_user_calendar.php');
         require_once(__DIR__ . '/kolab_invitation_calendar.php');
 
         $this->cal     = $cal;
         $this->rc      = $cal->rc;
         $this->storage = new kolab_storage();
 
         $this->cal->register_action('push-freebusy', [$this, 'push_freebusy']);
         $this->cal->register_action('calendar-acl', [$this, 'calendar_acl']);
 
         $this->freebusy_trigger = $this->rc->config->get('calendar_freebusy_trigger', false);
 
         if (!$this->rc->config->get('kolab_freebusy_server', false)) {
             $this->freebusy = false;
         }
 
         if (kolab_storage::$version == '2.0') {
             $this->alarm_types    = ['DISPLAY'];
             $this->alarm_absolute = false;
         }
 
         // get configuration for the Bonnie API
         $this->bonnie_api = libkolab::get_bonnie_api();
 
         // calendar uses fully encoded identifiers
         kolab_storage::$encode_ids = true;
     }
 
     /**
      * Read available calendars from server
      */
     protected function _read_calendars()
     {
         // already read sources
         if (isset($this->calendars)) {
             return $this->calendars;
         }
 
         // get all folders that have "event" type, sorted by namespace/name
         $folders = $this->storage->sort_folders(
             $this->storage->get_folders('event') + kolab_storage::get_user_folders('event', true)
         );
 
         $this->calendars = [];
 
         foreach ($folders as $folder) {
             $calendar = $this->_to_calendar($folder);
             if ($calendar->ready) {
                 $this->calendars[$calendar->id] = $calendar;
                 if ($calendar->editable) {
                     $this->has_writeable = true;
                 }
             }
         }
 
         return $this->calendars;
     }
 
     /**
      * Convert kolab_storage_folder into kolab_calendar
      */
     protected function _to_calendar($folder)
     {
         if ($folder instanceof kolab_calendar) {
             return $folder;
         }
 
         if ($folder instanceof kolab_storage_folder_user) {
             $calendar = new kolab_user_calendar($folder, $this->cal);
             $calendar->subscriptions = count($folder->children) > 0;
         }
         else {
             $calendar = new kolab_calendar($folder->name, $this->cal);
         }
 
         return $calendar;
     }
 
     /**
      * Get a list of available calendars from this source
      *
      * @param int    $filter Bitmask defining filter criterias
      * @param object $tree   Reference to hierarchical folder tree object
      *
      * @return array List of calendars
      */
     public function list_calendars($filter = 0, &$tree = null)
     {
         $this->_read_calendars();
 
         // attempt to create a default calendar for this user
         if (!$this->has_writeable) {
             if ($this->create_calendar(['name' => 'Calendar', 'color' => 'cc0000'])) {
                 unset($this->calendars);
                 $this->_read_calendars();
             }
         }
 
         $delim     = $this->rc->get_storage()->get_hierarchy_delimiter();
         $folders   = $this->filter_calendars($filter);
         $calendars = [];
 
         // include virtual folders for a full folder tree
         if (!is_null($tree)) {
             $folders = $this->storage->folder_hierarchy($folders, $tree);
         }
 
         $parents = array_keys($this->calendars);
 
         foreach ($folders as $id => $cal) {
             $imap_path = explode($delim, $cal->name);
 
             // find parent
             do {
                 array_pop($imap_path);
                 $parent_id = $this->storage->folder_id(join($delim, $imap_path));
             }
             while (count($imap_path) > 1 && !in_array($parent_id, $parents));
 
             // restore "real" parent ID
             if ($parent_id && !in_array($parent_id, $parents)) {
                 $parent_id = $this->storage->folder_id($cal->get_parent());
             }
 
             $parents[] = $cal->id;
 
             if ($cal->virtual) {
                 $calendars[$cal->id] = [
                     'id'       => $cal->id,
                     'name'     => $cal->get_name(),
                     'listname' => $cal->get_foldername(),
                     'editname' => $cal->get_foldername(),
                     'virtual'  => true,
                     'editable' => false,
                     'group'    => $cal->get_namespace(),
                 ];
             }
             else {
                 // additional folders may come from kolab_storage::folder_hierarchy() above
                 // make sure we deal with kolab_calendar instances
                 $cal = $this->_to_calendar($cal);
                 $this->calendars[$cal->id] = $cal;
 
                 $is_user = ($cal instanceof kolab_user_calendar);
 
                 $calendars[$cal->id] = [
                     'id'        => $cal->id,
                     'name'      => $cal->get_name(),
                     'listname'  => $cal->get_foldername(),
                     'editname'  => $cal->get_foldername(),
                     'title'     => $cal->get_title(),
                     'color'     => $cal->get_color(),
                     'editable'  => $cal->editable,
                     'group'     => $is_user ? 'other user' : $cal->get_namespace(),
                     'active'    => $cal->is_active(),
                     'owner'     => $cal->get_owner(),
                     'removable' => !$cal->default,
                 ];
 
                 if (!$is_user) {
                     $calendars[$cal->id] += [
                         'default'    => $cal->default,
                         'rights'     => $cal->rights,
                         'showalarms' => $cal->alarms,
                         'history'    => !empty($this->bonnie_api),
                         'children'   => true,  // TODO: determine if that folder indeed has child folders
                         'parent'     => $parent_id,
                         'subtype'    => $cal->subtype,
                         'caldavurl'  => $cal->get_caldav_url(),
                     ];
                 }
             }
 
             if ($cal->subscriptions) {
                 $calendars[$cal->id]['subscribed'] = $cal->is_subscribed();
             }
         }
 
         // list virtual calendars showing invitations
         if ($this->rc->config->get('kolab_invitation_calendars') && !($filter & self::FILTER_INSERTABLE)) {
             foreach ([self::INVITATIONS_CALENDAR_PENDING, self::INVITATIONS_CALENDAR_DECLINED] as $id) {
                 $cal = new kolab_invitation_calendar($id, $this->cal);
                 if (!($filter & self::FILTER_ACTIVE) || $cal->is_active()) {
                     $calendars[$id] = [
                         'id'         => $cal->id,
                         'name'       => $cal->get_name(),
                         'listname'   => $cal->get_name(),
                         'editname'   => $cal->get_foldername(),
                         'title'      => $cal->get_title(),
                         'color'      => $cal->get_color(),
                         'editable'   => $cal->editable,
                         'rights'     => $cal->rights,
                         'showalarms' => $cal->alarms,
                         'history'    => !empty($this->bonnie_api),
                         'group'      => 'x-invitations',
                         'default'    => false,
                         'active'     => $cal->is_active(),
                         'owner'      => $cal->get_owner(),
                         'children'   => false,
                         'counts'     => $id == self::INVITATIONS_CALENDAR_PENDING,
                     ];
 
 
                     if (is_object($tree)) {
                         $tree->children[] = $cal;
                     }
                 }
             }
         }
 
         // append the virtual birthdays calendar
         if ($this->rc->config->get('calendar_contact_birthdays', false) && !($filter & self::FILTER_INSERTABLE)) {
             $id    = self::BIRTHDAY_CALENDAR_ID;
             $prefs = $this->rc->config->get('kolab_calendars', []);  // read local prefs
 
             if (!($filter & self::FILTER_ACTIVE) || !empty($prefs[$id]['active'])) {
                 $calendars[$id] = [
                     'id'         => $id,
                     'name'       => $this->cal->gettext('birthdays'),
                     'listname'   => $this->cal->gettext('birthdays'),
                     'color'      => !empty($prefs[$id]['color']) ? $prefs[$id]['color'] : '87CEFA',
                     'active'     => !empty($prefs[$id]['active']),
                     'showalarms' => (bool) $this->rc->config->get('calendar_birthdays_alarm_type'),
                     'group'      => 'x-birthdays',
                     'editable'   => false,
                     'default'    => false,
                     'children'   => false,
                     'history'    => false,
                 ];
             }
         }
 
         return $calendars;
     }
 
     /**
      * Get list of calendars according to specified filters
      *
      * @param int Bitmask defining restrictions. See FILTER_* constants for possible values.
      *
      * @return array List of calendars
      */
     protected function filter_calendars($filter)
     {
         $this->_read_calendars();
 
         $calendars = [];
 
         $plugin = $this->rc->plugins->exec_hook('calendar_list_filter', [
             'list'      => $this->calendars,
             'calendars' => $calendars,
             'filter'    => $filter,
         ]);
 
         if ($plugin['abort']) {
             return $plugin['calendars'];
         }
 
         $personal = $filter & self::FILTER_PERSONAL;
         $shared   = $filter & self::FILTER_SHARED;
 
         foreach ($this->calendars as $cal) {
             if (!$cal->ready) {
                 continue;
             }
             if (($filter & self::FILTER_WRITEABLE) && !$cal->editable) {
                 continue;
             }
             if (($filter & self::FILTER_INSERTABLE) && !$cal->editable) {
                 continue;
             }
             if (($filter & self::FILTER_ACTIVE) && !$cal->is_active()) {
                 continue;
             }
             if (($filter & self::FILTER_PRIVATE) && $cal->subtype != 'private') {
                 continue;
             }
             if (($filter & self::FILTER_CONFIDENTIAL) && $cal->subtype != 'confidential') {
                 continue;
             }
             if ($personal || $shared) {
                 $ns = $cal->get_namespace();
                 if (!(($personal && $ns == 'personal') || ($shared && $ns == 'shared'))) {
                     continue;
                 }
             }
 
             $calendars[$cal->id] = $cal;
         }
 
         return $calendars;
     }
 
     /**
      * Get the kolab_calendar instance for the given calendar ID
      *
      * @param string Calendar identifier (encoded imap folder name)
      *
      * @return kolab_calendar Object nor null if calendar doesn't exist
      */
     public function get_calendar($id)
     {
         $this->_read_calendars();
 
         // create calendar object if necesary
         if (empty($this->calendars[$id])) {
             if (in_array($id, [self::INVITATIONS_CALENDAR_PENDING, self::INVITATIONS_CALENDAR_DECLINED])) {
                 return new kolab_invitation_calendar($id, $this->cal);
             }
 
             // for unsubscribed calendar folders
             if ($id !== self::BIRTHDAY_CALENDAR_ID) {
                 $calendar = kolab_calendar::factory($id, $this->cal);
                 if ($calendar->ready) {
                     $this->calendars[$calendar->id] = $calendar;
                 }
             }
         }
 
         return !empty($this->calendars[$id]) ? $this->calendars[$id] : null;
     }
 
     /**
      * Create a new calendar assigned to the current user
      *
      * @param array Hash array with calendar properties
      *    name: Calendar name
      *   color: The color of the calendar
      *
      * @return mixed ID of the calendar on success, False on error
      */
     public function create_calendar($prop)
     {
         $prop['type']       = 'event';
         $prop['active']     = true;
         $prop['subscribed'] = true;
 
         $folder = $this->storage->folder_update($prop);
 
         if ($folder === false) {
             $this->last_error = $this->cal->gettext($this->storage->last_error);
             return false;
         }
 
         // create ID
         $id = $this->storage->folder_id($folder);
 
         // save color in user prefs (temp. solution)
         $prefs['kolab_calendars'] = $this->rc->config->get('kolab_calendars', []);
 
         if (isset($prop['color'])) {
             $prefs['kolab_calendars'][$id]['color'] = $prop['color'];
         }
 
         if (isset($prop['showalarms'])) {
             $prefs['kolab_calendars'][$id]['showalarms'] = !empty($prop['showalarms']);
         }
 
         if (!empty($prefs['kolab_calendars'][$id])) {
             $this->rc->user->save_prefs($prefs);
         }
 
         return $id;
     }
 
     /**
      * Update properties of an existing calendar
      *
      * @see calendar_driver::edit_calendar()
      */
     public function edit_calendar($prop)
     {
         if (!empty($prop['id']) && ($cal = $this->get_calendar($prop['id']))) {
             $id = $cal->update($prop);
         }
         else {
             $id = $prop['id'];
         }
 
         // fallback to local prefs
         $prefs['kolab_calendars'] = $this->rc->config->get('kolab_calendars', []);
         unset($prefs['kolab_calendars'][$prop['id']]['color'], $prefs['kolab_calendars'][$prop['id']]['showalarms']);
 
         if (isset($prop['color'])) {
             $prefs['kolab_calendars'][$id]['color'] = $prop['color'];
         }
 
         if (isset($prop['showalarms']) && $id == self::BIRTHDAY_CALENDAR_ID) {
             $prefs['calendar_birthdays_alarm_type'] = $prop['showalarms'] ? $this->alarm_types[0] : '';
         }
         else if (isset($prop['showalarms'])) {
             $prefs['kolab_calendars'][$id]['showalarms'] = !empty($prop['showalarms']);
         }
 
         if (!empty($prefs['kolab_calendars'][$id])) {
             $this->rc->user->save_prefs($prefs);
         }
 
         return true;
     }
 
     /**
      * Set active/subscribed state of a calendar
      *
      * @see calendar_driver::subscribe_calendar()
      */
     public function subscribe_calendar($prop)
     {
         if (!empty($prop['id']) && ($cal = $this->get_calendar($prop['id'])) && !empty($cal->storage)) {
             $ret = false;
             if (isset($prop['permanent'])) {
                 $ret |= $cal->storage->subscribe(intval($prop['permanent']));
             }
             if (isset($prop['active'])) {
                 $ret |= $cal->storage->activate(intval($prop['active']));
             }
 
             // apply to child folders, too
             if (!empty($prop['recursive'])) {
                 foreach ((array) $this->storage->list_folders($cal->storage->name, '*', 'event') as $subfolder) {
                     if (isset($prop['permanent'])) {
                         if ($prop['permanent']) {
                             $this->storage->folder_subscribe($subfolder);
                         }
                         else {
                             $this->storage->folder_unsubscribe($subfolder);
                         }
                     }
 
                     if (isset($prop['active'])) {
                         if ($prop['active']) {
                             $this->storage->folder_activate($subfolder);
                         }
                         else {
                             $this->storage->folder_deactivate($subfolder);
                         }
                     }
                 }
             }
             return $ret;
         }
         else {
             // save state in local prefs
             $prefs['kolab_calendars'] = $this->rc->config->get('kolab_calendars', []);
             $prefs['kolab_calendars'][$prop['id']]['active'] = !empty($prop['active']);
             $this->rc->user->save_prefs($prefs);
             return true;
         }
 
         return false;
     }
 
     /**
      * Delete the given calendar with all its contents
      *
      * @see calendar_driver::delete_calendar()
      */
     public function delete_calendar($prop)
     {
         if (!empty($prop['id']) && ($cal = $this->get_calendar($prop['id']))) {
             $folder = $cal->get_realname();
 
             // TODO: unsubscribe if no admin rights
             if ($this->storage->folder_delete($folder)) {
                 // remove color in user prefs (temp. solution)
                 $prefs['kolab_calendars'] = $this->rc->config->get('kolab_calendars', []);
                 unset($prefs['kolab_calendars'][$prop['id']]);
 
                 $this->rc->user->save_prefs($prefs);
                 return true;
             }
             else {
                 $this->last_error = $this->storage->last_error;
             }
         }
 
         return false;
     }
 
     /**
      * Search for shared or otherwise not listed calendars the user has access
      *
      * @param string Search string
      * @param string Section/source to search
      *
      * @return array List of calendars
      */
     public function search_calendars($query, $source)
     {
         if (!$this->storage->setup()) {
             return [];
         }
 
         $this->calendars = [];
         $this->search_more_results = false;
 
         // find unsubscribed IMAP folders that have "event" type
         if ($source == 'folders') {
             foreach ((array) $this->storage->search_folders('event', $query, ['other']) as $folder) {
                 $calendar = new kolab_calendar($folder->name, $this->cal);
                 $this->calendars[$calendar->id] = $calendar;
             }
         }
         // find other user's virtual calendars
         else if ($source == 'users') {
             // we have slightly more space, so display twice the number
             $limit = $this->rc->config->get('autocomplete_max', 15) * 2;
 
             foreach ($this->storage->search_users($query, 0, [], $limit, $count) as $user) {
                 $calendar = new kolab_user_calendar($user, $this->cal);
                 $this->calendars[$calendar->id] = $calendar;
 
                 // search for calendar folders shared by this user
                 foreach ($this->storage->list_user_folders($user, 'event', false) as $foldername) {
                     $cal = new kolab_calendar($foldername, $this->cal);
                     $this->calendars[$cal->id] = $cal;
                     $calendar->subscriptions = true;
                 }
             }
 
             if ($count > $limit) {
                 $this->search_more_results = true;
             }
         }
 
         // don't list the birthday calendar
         $this->rc->config->set('calendar_contact_birthdays', false);
         $this->rc->config->set('kolab_invitation_calendars', false);
 
         return $this->list_calendars();
     }
 
     /**
      * Fetch a single event
      *
      * @see calendar_driver::get_event()
      * @return array Hash array with event properties, false if not found
      */
     public function get_event($event, $scope = 0, $full = false)
     {
         if (is_array($event)) {
             $id  = !empty($event['id']) ? $event['id'] : $event['uid'];
             $cal = $event['calendar'];
 
             // we're looking for a recurring instance: expand the ID to our internal convention for recurring instances
             if (empty($event['id']) && !empty($event['_instance'])) {
                 $id .= '-' . $event['_instance'];
             }
         }
         else {
             $id = $event;
         }
 
         if (!empty($cal)) {
             if ($storage = $this->get_calendar($cal)) {
                 $result = $storage->get_event($id);
                 return self::to_rcube_event($result);
             }
 
             // get event from the address books birthday calendar
             if ($cal == self::BIRTHDAY_CALENDAR_ID) {
                 return $this->get_birthday_event($id);
             }
         }
         // iterate over all calendar folders and search for the event ID
         else {
             foreach ($this->filter_calendars($scope) as $calendar) {
                 if ($result = $calendar->get_event($id)) {
                     return self::to_rcube_event($result);
                 }
             }
         }
 
         return false;
     }
 
     /**
      * Add a single event to the database
      *
      * @see calendar_driver::new_event()
      */
     public function new_event($event)
     {
         if (!$this->validate($event)) {
             return false;
         }
 
         $event = self::from_rcube_event($event);
 
         if (!$event['calendar']) {
             $this->_read_calendars();
             $cal_ids = array_keys($this->calendars);
             $event['calendar'] = reset($cal_ids);
         }
 
         if ($storage = $this->get_calendar($event['calendar'])) {
             // if this is a recurrence instance, append as exception to an already existing object for this UID
             if (!empty($event['recurrence_date']) && ($master = $storage->get_event($event['uid']))) {
                 self::add_exception($master, $event);
                 $success = $storage->update_event($master);
             }
             else {
                 $success = $storage->insert_event($event);
             }
 
             if ($success && $this->freebusy_trigger) {
                 $this->rc->output->command('plugin.ping_url', ['action' => 'calendar/push-freebusy', 'source' => $storage->id]);
                 $this->freebusy_trigger = false; // disable after first execution (#2355)
             }
 
             return $success;
         }
 
         return false;
     }
 
     /**
      * Update an event entry with the given data
      *
      * @see calendar_driver::new_event()
      * @return bool True on success, False on error
      */
     public function edit_event($event)
     {
         if (!($storage = $this->get_calendar($event['calendar']))) {
             return false;
         }
 
         return $this->update_event(self::from_rcube_event($event, $storage->get_event($event['id'])));
     }
 
     /**
      * Extended event editing with possible changes to the argument
      *
      * @param array  Hash array with event properties
      * @param string New participant status
      * @param array  List of hash arrays with updated attendees
      *
      * @return bool True on success, False on error
      */
     public function edit_rsvp(&$event, $status, $attendees)
     {
         $update_event = $event;
 
         // apply changes to master (and all exceptions)
         if ($event['_savemode'] == 'all' && !empty($event['recurrence_id'])) {
             if ($storage = $this->get_calendar($event['calendar'])) {
                 $update_event = $storage->get_event($event['recurrence_id']);
                 $update_event['_savemode'] = $event['_savemode'];
                 $update_event['id'] = $update_event['uid'];
                 unset($update_event['recurrence_id']);
                 calendar::merge_attendee_data($update_event, $attendees);
             }
         }
 
         if ($ret = $this->update_attendees($update_event, $attendees)) {
             // replace with master event (for iTip reply)
             $event = self::to_rcube_event($update_event);
 
             // re-assign to the according (virtual) calendar
             if ($this->rc->config->get('kolab_invitation_calendars')) {
                 if (strtoupper($status) == 'DECLINED') {
                     $event['calendar'] = self::INVITATIONS_CALENDAR_DECLINED;
                 }
                 else if (strtoupper($status) == 'NEEDS-ACTION') {
                     $event['calendar'] = self::INVITATIONS_CALENDAR_PENDING;
                 }
                 else if (!empty($event['_folder_id'])) {
                     $event['calendar'] = $event['_folder_id'];
                 }
             }
         }
 
         return $ret;
     }
 
     /**
      * Update the participant status for the given attendees
      *
      * @see calendar_driver::update_attendees()
      */
     public function update_attendees(&$event, $attendees)
     {
         // for this-and-future updates, merge the updated attendees onto all exceptions in range
         if (
             ($event['_savemode'] == 'future' && !empty($event['recurrence_id']))
             || (!empty($event['recurrence']) && empty($event['recurrence_id']))
         ) {
             if (!($storage = $this->get_calendar($event['calendar']))) {
                 return false;
             }
 
             // load master event
             $master = !empty($event['recurrence_id']) ? $storage->get_event($event['recurrence_id']) : $event;
 
             // apply attendee update to each existing exception
             if (!empty($master['recurrence']) && !empty($master['recurrence']['EXCEPTIONS'])) {
                 $saved = false;
                 foreach ($master['recurrence']['EXCEPTIONS'] as $i => $exception) {
                     // merge the new event properties onto future exceptions
                     if ($exception['_instance'] >= strval($event['_instance'])) {
                         calendar::merge_attendee_data($master['recurrence']['EXCEPTIONS'][$i], $attendees);
                     }
                     // update a specific instance
                     if ($exception['_instance'] == $event['_instance'] && $exception['thisandfuture']) {
                         $saved = true;
                     }
                 }
 
                 // add the given event as new exception
                 if (!$saved && $event['id'] != $master['id']) {
                     $event['thisandfuture'] = true;
                     $master['recurrence']['EXCEPTIONS'][] = $event;
                 }
 
                 // set link to top-level exceptions
                 $master['exceptions'] = &$master['recurrence']['EXCEPTIONS'];
 
                 return $this->update_event($master);
             }
         }
 
         // just update the given event (instance)
         return $this->update_event($event);
     }
 
     /**
      * Move a single event
      *
      * @see calendar_driver::move_event()
      * @return boolean True on success, False on error
      */
     public function move_event($event)
     {
         if (($storage = $this->get_calendar($event['calendar'])) && ($ev = $storage->get_event($event['id']))) {
             unset($ev['sequence']);
             self::clear_attandee_noreply($ev);
 
             return $this->update_event($event + $ev);
         }
 
         return false;
     }
 
     /**
      * Resize a single event
      *
      * @see calendar_driver::resize_event()
      * @return boolean True on success, False on error
      */
     public function resize_event($event)
     {
         if (($storage = $this->get_calendar($event['calendar'])) && ($ev = $storage->get_event($event['id']))) {
             unset($ev['sequence']);
             self::clear_attandee_noreply($ev);
 
             return $this->update_event($event + $ev);
         }
 
         return false;
     }
 
     /**
      * Remove a single event
      *
      * @param array Hash array with event properties:
      *      id: Event identifier
      * @param bool  Remove record(s) irreversible (mark as deleted otherwise)
      *
      * @return bool True on success, False on error
      */
     public function remove_event($event, $force = true)
     {
         $ret      = true;
         $success  = false;
-        $savemode = isset($event['_savemode']) ? $event['_savemode'] : null;
+        $savemode = $event['_savemode'] ?? null;
 
         if (!$force) {
             unset($event['attendees']);
             $this->rc->session->remove('calendar_event_undo');
             $this->rc->session->remove('calendar_restore_event_data');
             $sess_data = $event;
         }
 
         if (($storage = $this->get_calendar($event['calendar'])) && ($event = $storage->get_event($event['id']))) {
             $event['_savemode'] = $savemode;
             $decline  = !empty($event['_decline']);
             $savemode = 'all';
             $master   = $event;
 
             // read master if deleting a recurring event
             if (!empty($event['recurrence']) || !empty($event['recurrence_id']) || !empty($event['isexception'])) {
                 $master = $storage->get_event($event['uid']);
 
                 if (!empty($event['_savemode'])) {
                     $savemode = $event['_savemode'];
                 }
                 else if (!empty($event['_instance']) || !empty($event['isexception'])) {
                     $savemode = 'current';
                 }
 
                 // force 'current' mode for single occurrences stored as exception
                 if (empty($event['recurrence']) && empty($event['recurrence_id']) && !empty($event['isexception'])) {
                     $savemode = 'current';
                 }
             }
 
             // removing an exception instance
             if ((!empty($event['recurrence_id']) || !empty($event['isexception'])) && !empty($master['exceptions'])) {
                 foreach ($master['exceptions'] as $i => $exception) {
-                    if ($exception['_instance'] == $event['_instance']) {
+                    if (libcalendaring::is_recurrence_exception($event, $exception)) {
                         unset($master['exceptions'][$i]);
                         // set event date back to the actual occurrence
                         if (!empty($exception['recurrence_date'])) {
                             $event['start'] = $exception['recurrence_date'];
                         }
                     }
                 }
 
                 if (!empty($master['recurrence'])) {
                     $master['recurrence']['EXCEPTIONS'] = &$master['exceptions'];
                 }
             }
 
             switch ($savemode) {
             case 'current':
                 $_SESSION['calendar_restore_event_data'] = $master;
 
                 // remove the matching RDATE entry
                 if (!empty($master['recurrence']['RDATE'])) {
                     foreach ($master['recurrence']['RDATE'] as $j => $rdate) {
                         if ($rdate->format('Ymd') == $event['start']->format('Ymd')) {
                             unset($master['recurrence']['RDATE'][$j]);
                             break;
                         }
                     }
                 }
 
                 // add exception to master event
                 $master['recurrence']['EXDATE'][] = $event['start'];
 
                 $success = $storage->update_event($master);
                 break;
 
             case 'future':
                 $master['_instance'] = libcalendaring::recurrence_instance_identifier($master);
                 if ($master['_instance'] != $event['_instance']) {
                     $_SESSION['calendar_restore_event_data'] = $master;
 
                     // set until-date on master event
                     $master['recurrence']['UNTIL'] = clone $event['start'];
                     $master['recurrence']['UNTIL']->sub(new DateInterval('P1D'));
                     unset($master['recurrence']['COUNT']);
 
                     // if all future instances are deleted, remove recurrence rule entirely (bug #1677)
                     if ($master['recurrence']['UNTIL']->format('Ymd') == $master['start']->format('Ymd')) {
                         $master['recurrence'] = [];
                     }
                     // remove matching RDATE entries
                     else if (!empty($master['recurrence']['RDATE'])) {
                         foreach ($master['recurrence']['RDATE'] as $j => $rdate) {
                             if ($rdate->format('Ymd') == $event['start']->format('Ymd')) {
                                 $master['recurrence']['RDATE'] = array_slice($master['recurrence']['RDATE'], 0, $j);
                                 break;
                             }
                         }
                     }
 
                     $success = $storage->update_event($master);
                     $ret = $master['uid'];
                     break;
                 }
 
             default:  // 'all' is default
                 // removing the master event with loose exceptions (not recurring though)
                 if (!empty($event['recurrence_date']) && empty($master['recurrence']) && !empty($master['exceptions'])) {
                     // make the first exception the new master
                     $newmaster = array_shift($master['exceptions']);
                     $newmaster['exceptions']   = $master['exceptions'];
                     $newmaster['_attachments'] = $master['_attachments'];
                     $newmaster['_mailbox']     = $master['_mailbox'];
                     $newmaster['_msguid']      = $master['_msguid'];
 
                     $success = $storage->update_event($newmaster);
                 }
                 else if ($decline && $this->rc->config->get('kolab_invitation_calendars')) {
                     // don't delete but set PARTSTAT=DECLINED
                     if ($this->cal->lib->set_partstat($master, 'DECLINED')) {
                         $success = $storage->update_event($master);
                     }
                 }
 
                 if (!$success) {
                     $success = $storage->delete_event($master, $force);
                 }
                 break;
             }
         }
 
         if ($success && !$force) {
             if (!empty($master['_folder_id'])) {
                 $sess_data['_folder_id'] = $master['_folder_id'];
             }
             $_SESSION['calendar_event_undo'] = ['ts' => time(), 'data' => $sess_data];
         }
 
         if ($success && $this->freebusy_trigger) {
             $this->rc->output->command('plugin.ping_url', [
                     'action' => 'calendar/push-freebusy',
                     // _folder_id may be set by invitations calendar
                     'source' => !empty($master['_folder_id']) ? $master['_folder_id'] : $storage->id,
             ]);
         }
 
         return $success ? $ret : false;
     }
 
     /**
      * Restore a single deleted event
      *
      * @param array Hash array with event properties:
      *                    id: Event identifier
      *              calendar: Event calendar
      *
      * @return bool True on success, False on error
      */
     public function restore_event($event)
     {
         if ($storage = $this->get_calendar($event['calendar'])) {
             if (!empty($_SESSION['calendar_restore_event_data'])) {
                 $success = $storage->update_event($event = $_SESSION['calendar_restore_event_data']);
             }
             else {
                 $success = $storage->restore_event($event);
             }
 
             if ($success && $this->freebusy_trigger) {
                 $this->rc->output->command('plugin.ping_url', [
                         'action' => 'calendar/push-freebusy',
                         // _folder_id may be set by invitations calendar
                         'source' => !empty($event['_folder_id']) ? $event['_folder_id'] : $storage->id,
                 ]);
             }
 
             return $success;
         }
 
         return false;
     }
 
     /**
      * Wrapper to update an event object depending on the given savemode
      */
     protected function update_event($event)
     {
         if (!($storage = $this->get_calendar($event['calendar']))) {
             return false;
         }
 
         // move event to another folder/calendar
         if (!empty($event['_fromcalendar']) && $event['_fromcalendar'] != $event['calendar']) {
             if (!($fromcalendar = $this->get_calendar($event['_fromcalendar']))) {
                 return false;
             }
 
             $old = $fromcalendar->get_event($event['id']);
 
             if ($event['_savemode'] != 'new') {
                 if (!$fromcalendar->storage->move($old['uid'], $storage->storage)) {
                     return false;
                 }
 
                 $fromcalendar = $storage;
             }
         }
         else {
             $fromcalendar = $storage;
         }
 
         $success  = false;
         $savemode = 'all';
 
         $old = $master = $storage->get_event($event['id']);
 
         if (!$old || empty($old['start'])) {
             rcube::raise_error([
                     'code' => 600, 'file' => __FILE__, 'line' => __LINE__,
                     'message' => "Failed to load event object to update: id=" . $event['id']
                 ],
                 true, false
             );
             return false;
         }
 
         // modify a recurring event, check submitted savemode to do the right things
         if (!empty($old['recurrence']) || !empty($old['recurrence_id']) || !empty($old['isexception'])) {
             $master = $storage->get_event($old['uid']);
 
             if (!empty($event['_savemode'])) {
                 $savemode = $event['_savemode'];
             }
             else {
                 $savemode = (!empty($old['recurrence_id']) || !empty($old['isexception'])) ? 'current' : 'all';
             }
 
             // this-and-future on the first instance equals to 'all'
             if ($savemode == 'future' && !empty($master['start'])
                 && $old['_instance'] == libcalendaring::recurrence_instance_identifier($master)
             ) {
                 $savemode = 'all';
             }
             // force 'current' mode for single occurrences stored as exception
             else if (empty($old['recurrence']) && empty($old['recurrence_id']) && !empty($old['isexception'])) {
                 $savemode = 'current';
             }
 
             // Stick to the master timezone for all occurrences (Bifrost#T104637)
-            $master_tz = $master['start']->getTimezone();
-            $event_tz  = $event['start']->getTimezone();
+            if (empty($master['allday']) || !empty($event['allday'])) {
+                $master_tz = $master['start']->getTimezone();
+                $event_tz  = $event['start']->getTimezone();
 
-            if ($master_tz->getName() != $event_tz->getName()) {
-                $event['start']->setTimezone($master_tz);
-                $event['end']->setTimezone($master_tz);
+                if ($master_tz->getName() != $event_tz->getName()) {
+                    $event['start']->setTimezone($master_tz);
+                    $event['end']->setTimezone($master_tz);
+                }
             }
         }
 
         // check if update affects scheduling and update attendee status accordingly
         $reschedule = $this->check_scheduling($event, $old, true);
 
         // keep saved exceptions (not submitted by the client)
         if (!empty($old['recurrence']['EXDATE']) && !isset($event['recurrence']['EXDATE'])) {
             $event['recurrence']['EXDATE'] = $old['recurrence']['EXDATE'];
         }
 
         if (isset($event['recurrence']['EXCEPTIONS'])) {
             // exceptions already provided (e.g. from iCal import)
             $with_exceptions = true;
         }
         else if (!empty($old['recurrence']['EXCEPTIONS'])) {
             $event['recurrence']['EXCEPTIONS'] = $old['recurrence']['EXCEPTIONS'];
         }
         else if (!empty($old['exceptions'])) {
             $event['exceptions'] = $old['exceptions'];
         }
 
         // remove some internal properties which should not be saved
         unset($event['_savemode'], $event['_fromcalendar'], $event['_identity'], $event['_owner'],
             $event['_notify'], $event['_method'], $event['_sender'], $event['_sender_utf'], $event['_size']
         );
 
         switch ($savemode) {
         case 'new':
             // save submitted data as new (non-recurring) event
             $event['recurrence'] = [];
             $event['_copyfrom']  = $master['_msguid'];
             $event['_mailbox']   = $master['_mailbox'];
             $event['uid']        = $this->cal->generate_uid();
 
             unset($event['recurrence_id'], $event['recurrence_date'], $event['_instance'], $event['id']);
 
             // copy attachment metadata to new event
             $event = self::from_rcube_event($event, $master);
 
             self::clear_attandee_noreply($event);
             if ($success = $storage->insert_event($event)) {
                 $success = $event['uid'];
             }
             break;
 
         case 'future':
             // create a new recurring event
             $event['_copyfrom'] = $master['_msguid'];
             $event['_mailbox']  = $master['_mailbox'];
             $event['uid']       = $this->cal->generate_uid();
 
             unset($event['recurrence_id'], $event['recurrence_date'], $event['_instance'], $event['id']);
 
             // copy attachment metadata to new event
             $event = self::from_rcube_event($event, $master);
 
             // remove recurrence exceptions on re-scheduling
             if ($reschedule) {
                 unset($event['recurrence']['EXCEPTIONS'], $event['exceptions'], $master['recurrence']['EXDATE']);
             }
-            else if (isset($event['recurrence']['EXCEPTIONS']) && is_array($event['recurrence']['EXCEPTIONS'])) {
-                // only keep relevant exceptions
-                $event['recurrence']['EXCEPTIONS'] = array_filter(
-                    $event['recurrence']['EXCEPTIONS'],
-                    function($exception) use ($event) {
-                        return $exception['start'] > $event['start'];
-                    }
-                );
+            else {
+                if (isset($event['recurrence']['EXCEPTIONS']) && is_array($event['recurrence']['EXCEPTIONS'])) {
+                    // only keep relevant exceptions
+                    $event['recurrence']['EXCEPTIONS'] = array_filter(
+                        $event['recurrence']['EXCEPTIONS'],
+                        function($exception) use ($event) {
+                            return $exception['start'] > $event['start'];
+                        }
+                    );
+
+                    // set link to top-level exceptions
+                    $event['exceptions'] = &$event['recurrence']['EXCEPTIONS'];
+                }
+
                 if (isset($event['recurrence']['EXDATE']) && is_array($event['recurrence']['EXDATE'])) {
                     $event['recurrence']['EXDATE'] = array_filter(
                         $event['recurrence']['EXDATE'],
                         function($exdate) use ($event) {
                             return $exdate > $event['start'];
                         }
                     );
                 }
-                // set link to top-level exceptions
-                $event['exceptions'] = &$event['recurrence']['EXCEPTIONS'];
             }
 
             // compute remaining occurrences
             if ($event['recurrence']['COUNT']) {
                 if (empty($old['_count'])) {
                     $old['_count'] = $this->get_recurrence_count($master, $old['start']);
                 }
                 $event['recurrence']['COUNT'] -= intval($old['_count']);
             }
 
             // remove fixed weekday when date changed
             if ($old['start']->format('Y-m-d') != $event['start']->format('Y-m-d')) {
                 if (!empty($event['recurrence']['BYDAY']) && strlen($event['recurrence']['BYDAY']) == 2) {
                     unset($event['recurrence']['BYDAY']);
                 }
                 if (!empty($old['recurrence']['BYMONTH']) && $old['recurrence']['BYMONTH'] == $old['start']->format('n')) {
                     unset($event['recurrence']['BYMONTH']);
                 }
             }
 
             // set until-date on master event
             $master['recurrence']['UNTIL'] = clone $old['start'];
             $master['recurrence']['UNTIL']->sub(new DateInterval('P1D'));
             unset($master['recurrence']['COUNT']);
 
             // remove all exceptions after $event['start']
             if (isset($master['recurrence']['EXCEPTIONS']) && is_array($master['recurrence']['EXCEPTIONS'])) {
                 $master['recurrence']['EXCEPTIONS'] = array_filter(
                     $master['recurrence']['EXCEPTIONS'],
                     function($exception) use ($event) {
                         return $exception['start'] < $event['start'];
                     }
                 );
                 // set link to top-level exceptions
                 $master['exceptions'] = &$master['recurrence']['EXCEPTIONS'];
             }
 
             if (isset($master['recurrence']['EXDATE']) && is_array($master['recurrence']['EXDATE'])) {
                 $master['recurrence']['EXDATE'] = array_filter(
                     $master['recurrence']['EXDATE'],
                     function($exdate) use ($event) {
                         return $exdate < $event['start'];
                     }
                 );
             }
 
             // save new event
             if ($success = $storage->insert_event($event)) {
                 $success = $event['uid'];
 
                 // update master event (no rescheduling!)
                 self::clear_attandee_noreply($master);
                 $storage->update_event($master);
             }
             break;
 
         case 'current':
             // recurring instances shall not store recurrence rules and attachments
             $event['recurrence']    = [];
             $event['thisandfuture'] = $savemode == 'future';
             unset($event['attachments'], $event['id']);
 
             // increment sequence of this instance if scheduling is affected
             if ($reschedule) {
                 $event['sequence'] = max($old['sequence'] ?? 0, $master['sequence'] ?? 0) + 1;
             }
             else if (!isset($event['sequence'])) {
-                $event['sequence'] = !empty($old['sequence']) ? $old['sequence'] : $master['sequence'];
+                $event['sequence'] = !empty($old['sequence']) ? $old['sequence'] : $master['sequence'] ?? 1;
             }
 
             // save properties to a recurrence exception instance
             if (!empty($old['_instance']) && isset($master['recurrence']['EXCEPTIONS'])) {
                 if ($this->update_recurrence_exceptions($master, $event, $old, $savemode)) {
                     $success = $storage->update_event($master, $old['id']);
                     break;
                 }
             }
 
             $add_exception = true;
 
             // adjust matching RDATE entry if dates changed
             if (
                 !empty($master['recurrence']['RDATE'])
                 && ($old_date = $old['start']->format('Ymd')) != $event['start']->format('Ymd')
             ) {
                 foreach ($master['recurrence']['RDATE'] as $j => $rdate) {
                     if ($rdate->format('Ymd') == $old_date) {
                         $master['recurrence']['RDATE'][$j] = $event['start'];
                         sort($master['recurrence']['RDATE']);
                         $add_exception = false;
                         break;
                     }
                 }
             }
 
             // save as new exception to master event
             if ($add_exception) {
                 self::add_exception($master, $event, $old);
             }
 
             $success = $storage->update_event($master);
             break;
 
         default:  // 'all' is the default
             $event['id']  = $master['uid'];
             $event['uid'] = $master['uid'];
 
             // use start date from master but try to be smart on time or duration changes
             $old_start_date = $old['start']->format('Y-m-d');
             $old_start_time = !empty($old['allday']) ? '' : $old['start']->format('H:i');
             $old_duration   = self::event_duration($old['start'], $old['end'], !empty($old['allday']));
 
             $new_start_date = $event['start']->format('Y-m-d');
             $new_start_time = !empty($event['allday']) ? '' : $event['start']->format('H:i');
             $new_duration   = self::event_duration($event['start'], $event['end'], !empty($event['allday']));
 
             $diff = $old_start_date != $new_start_date || $old_start_time != $new_start_time || $old_duration != $new_duration;
             $date_shift = $old['start']->diff($event['start']);
 
             // shifted or resized
             if ($diff && ($old_start_date == $new_start_date || $old_duration == $new_duration)) {
                 $event['start'] = $master['start']->add($date_shift);
                 $event['end'] = clone $event['start'];
                 $event['end']->add(new DateInterval($new_duration));
 
                 // remove fixed weekday, will be re-set to the new weekday in kolab_calendar::update_event()
                 if ($old_start_date != $new_start_date && !empty($event['recurrence'])) {
                     if (!empty($event['recurrence']['BYDAY']) && strlen($event['recurrence']['BYDAY']) == 2)
                         unset($event['recurrence']['BYDAY']);
                     if (!empty($old['recurrence']['BYMONTH']) && $old['recurrence']['BYMONTH'] == $old['start']->format('n'))
                         unset($event['recurrence']['BYMONTH']);
                 }
             }
             // dates did not change, use the ones from master
             else if ($new_start_date . $new_start_time == $old_start_date . $old_start_time) {
                 $event['start'] = $master['start'];
                 $event['end'] = $master['end'];
             }
 
             // when saving an instance in 'all' mode, copy recurrence exceptions over
             if (!empty($old['recurrence_id'])) {
                 $event['recurrence']['EXCEPTIONS'] = $master['recurrence']['EXCEPTIONS'] ?? [];
                 $event['recurrence']['EXDATE']     = $master['recurrence']['EXDATE'] ?? [];
             }
             else if (!empty($master['_instance'])) {
                 $event['_instance']       = $master['_instance'];
                 $event['recurrence_date'] = $master['recurrence_date'];
             }
 
             // TODO: forward changes to exceptions (which do not yet have differing values stored)
-            if (!empty($event['recurrence']) && !empty($event['recurrence']['EXCEPTIONS']) && !$with_exceptions) {
+            if (!empty($event['recurrence']) && !empty($event['recurrence']['EXCEPTIONS']) && empty($with_exceptions)) {
                 // determine added and removed attendees
                 $old_attendees = $current_attendees = $added_attendees = [];
 
                 if (!empty($old['attendees'])) {
                     foreach ((array) $old['attendees'] as $attendee) {
                         $old_attendees[] = $attendee['email'];
                     }
                 }
 
                 if (!empty($event['attendees'])) {
                     foreach ((array) $event['attendees'] as $attendee) {
                         $current_attendees[] = $attendee['email'];
                         if (!in_array($attendee['email'], $old_attendees)) {
                             $added_attendees[] = $attendee;
                         }
                     }
                 }
 
                 $removed_attendees = array_diff($old_attendees, $current_attendees);
 
                 foreach ($event['recurrence']['EXCEPTIONS'] as $i => $exception) {
                     calendar::merge_attendee_data($event['recurrence']['EXCEPTIONS'][$i], $added_attendees, $removed_attendees);
                 }
 
                 // adjust recurrence-id when start changed and therefore the entire recurrence chain changes
                 if ($old_start_date != $new_start_date || $old_start_time != $new_start_time) {
                     $recurrence_id_format = libcalendaring::recurrence_id_format($event);
 
                     foreach ($event['recurrence']['EXCEPTIONS'] as $i => $exception) {
                         if (isset($exception['recurrence_date']) && $exception['recurrence_date'] instanceof DateTimeInterface) {
                             $recurrence_id = $exception['recurrence_date'];
                         }
                         else {
                             $recurrence_id = rcube_utils::anytodatetime($exception['_instance'], $old['start']->getTimezone());
                         }
 
                         if ($recurrence_id instanceof DateTimeInterface) {
                             $recurrence_id->add($date_shift);
                             $event['recurrence']['EXCEPTIONS'][$i]['recurrence_date'] = $recurrence_id;
                             $event['recurrence']['EXCEPTIONS'][$i]['_instance'] = $recurrence_id->format($recurrence_id_format);
                         }
                     }
                 }
 
                 // set link to top-level exceptions
                 $event['exceptions'] = &$event['recurrence']['EXCEPTIONS'];
             }
 
             // unset _dateonly flags in (cached) date objects
             unset($event['start']->_dateonly, $event['end']->_dateonly);
 
             $success = $storage->update_event($event) ? $event['id'] : false;  // return master UID
             break;
         }
 
         if ($success && $this->freebusy_trigger) {
             $this->rc->output->command('plugin.ping_url', [
                     'action' => 'calendar/push-freebusy',
                     'source' => $storage->id
             ]);
         }
 
         return $success;
     }
 
     /**
      * Calculate event duration, returns string in DateInterval format
      */
     protected static function event_duration($start, $end, $allday = false)
     {
         if ($allday) {
             $diff = $start->diff($end);
             return 'P' . $diff->days . 'D';
         }
 
         return 'PT' . ($end->format('U') - $start->format('U')) . 'S';
     }
 
     /**
      * Determine whether the current change affects scheduling and reset attendee status accordingly
      */
     protected function check_scheduling(&$event, $old, $update = true)
     {
         // skip this check when importing iCal/iTip events
         if (isset($event['sequence']) || !empty($event['_method'])) {
             return false;
         }
 
         // iterate through the list of properties considered 'significant' for scheduling
         $kolab_event = !empty($old['_formatobj']) ? $old['_formatobj'] : new kolab_format_event();
         $reschedule  = $kolab_event->check_rescheduling($event, $old);
 
         // reset all attendee status to needs-action (#4360)
         if ($update && $reschedule && !empty($event['attendees'])) {
             $is_organizer = false;
             $emails       = $this->cal->get_user_emails();
             $attendees    = $event['attendees'];
 
             foreach ($attendees as $i => $attendee) {
                 if ($attendee['role'] == 'ORGANIZER'
                     && !empty($attendee['email'])
                     && in_array(strtolower($attendee['email']), $emails)
                 ) {
                     $is_organizer = true;
                 }
                 else if ($attendee['role'] != 'ORGANIZER'
                     && $attendee['role'] != 'NON-PARTICIPANT'
                     && $attendee['status'] != 'DELEGATED'
                 ) {
                     $attendees[$i]['status'] = 'NEEDS-ACTION';
                     $attendees[$i]['rsvp'] = true;
                 }
             }
 
             // update attendees only if I'm the organizer
             if ($is_organizer || (!empty($event['organizer']) && in_array(strtolower($event['organizer']['email']), $emails))) {
                 $event['attendees'] = $attendees;
             }
         }
 
         return $reschedule;
     }
 
     /**
      * Apply the given changes to already existing exceptions
      */
     protected function update_recurrence_exceptions(&$master, $event, $old, $savemode)
     {
         $saved    = false;
         $existing = null;
 
         // determine added and removed attendees
         $added_attendees = $removed_attendees = [];
 
         if ($savemode == 'future') {
             $old_attendees = $current_attendees = [];
 
             if (!empty($old['attendees'])) {
                 foreach ((array) $old['attendees'] as $attendee) {
                     $old_attendees[] = $attendee['email'];
                 }
             }
 
             if (!empty($event['attendees'])) {
                 foreach ((array) $event['attendees'] as $attendee) {
                     $current_attendees[] = $attendee['email'];
                     if (!in_array($attendee['email'], $old_attendees)) {
                         $added_attendees[] = $attendee;
                     }
                 }
             }
 
             $removed_attendees = array_diff($old_attendees, $current_attendees);
         }
 
         foreach ($master['recurrence']['EXCEPTIONS'] as $i => $exception) {
             // update a specific instance
-            if ($exception['_instance'] == $old['_instance']) {
+            if (libcalendaring::is_recurrence_exception($old, $exception)) {
                 $existing = $i;
 
                 // check savemode against existing exception mode.
                 // if matches, we can update this existing exception
                 $thisandfuture = !empty($exception['thisandfuture']);
                 if ($thisandfuture === ($savemode == 'future')) {
                     $event['_instance']       = $old['_instance'];
-                    $event['thisandfuture']   = $old['thisandfuture'];
+                    $event['thisandfuture']   = !empty($old['thisandfuture']);
                     $event['recurrence_date'] = $old['recurrence_date'];
                     $master['recurrence']['EXCEPTIONS'][$i] = $event;
                     $saved = true;
                 }
             }
 
             // merge the new event properties onto future exceptions
-            if ($savemode == 'future' && $exception['_instance'] >= $old['_instance']) {
-                unset($event['thisandfuture']);
-                self::merge_exception_data($master['recurrence']['EXCEPTIONS'][$i], $event, ['attendees']);
+            if ($savemode == 'future') {
+                $exception_instance = libcalendaring::recurrence_instance_identifier($exception, true);
+                $old_instance = libcalendaring::recurrence_instance_identifier($old, true);
 
-                if (!empty($added_attendees) || !empty($removed_attendees)) {
-                    calendar::merge_attendee_data($master['recurrence']['EXCEPTIONS'][$i], $added_attendees, $removed_attendees);
+                if ($exception_instance >= $old_instance) {
+                    unset($event['thisandfuture']);
+                    self::merge_exception_data($master['recurrence']['EXCEPTIONS'][$i], $event, ['attendees']);
+
+                    if (!empty($added_attendees) || !empty($removed_attendees)) {
+                        calendar::merge_attendee_data($master['recurrence']['EXCEPTIONS'][$i], $added_attendees, $removed_attendees);
+                    }
                 }
             }
         }
 /*
         // we could not update the existing exception due to savemode mismatch...
         if (!$saved && isset($existing) && !empty($master['recurrence']['EXCEPTIONS'][$existing]['thisandfuture'])) {
             // ... try to move the existing this-and-future exception to the next occurrence
             foreach ($this->get_recurring_events($master, $existing['start']) as $candidate) {
                 // our old this-and-future exception is obsolete
                 if (!empty($candidate['thisandfuture'])) {
                     unset($master['recurrence']['EXCEPTIONS'][$existing]);
                     $saved = true;
                     break;
                 }
                 // this occurrence doesn't yet have an exception
                 else if (empty($candidate['isexception'])) {
                     $event['_instance'] = $candidate['_instance'];
                     $event['recurrence_date'] = $candidate['recurrence_date'];
                     $master['recurrence']['EXCEPTIONS'][$i] = $event;
                     $saved = true;
                     break;
                 }
             }
         }
 */
 
         // set link to top-level exceptions
         $master['exceptions'] = &$master['recurrence']['EXCEPTIONS'];
 
         // returning false here will add a new exception
         return $saved;
     }
 
     /**
      * Add or update the given event as an exception to $master
      */
     public static function add_exception(&$master, $event, $old = null)
     {
         if ($old) {
-            $event['_instance'] = $old['_instance'];
+            $event['_instance'] = $old['_instance'] ?? null;
             if (empty($event['recurrence_date'])) {
                 $event['recurrence_date'] = !empty($old['recurrence_date']) ? $old['recurrence_date'] : $old['start'];
             }
         }
         else if (empty($event['recurrence_date'])) {
             $event['recurrence_date'] = $event['start'];
         }
 
-        if (empty($event['_instance']) && $event['recurrence_date'] instanceof DateTimeInterface) {
-            $event['_instance'] = libcalendaring::recurrence_instance_identifier($event, !empty($master['allday']));
-        }
-
         if (!isset($master['exceptions'])) {
             if (isset($master['recurrence']['EXCEPTIONS'])) {
                 $master['exceptions'] = &$master['recurrence']['EXCEPTIONS'];
             }
             else {
                 $master['exceptions'] = [];
             }
         }
 
         $existing = false;
         foreach ($master['exceptions'] as $i => $exception) {
-            if ($exception['_instance'] == $event['_instance']) {
+            if (libcalendaring::is_recurrence_exception($event, $exception)) {
                 $master['exceptions'][$i] = $event;
                 $existing = true;
             }
         }
 
         if (!$existing) {
             $master['exceptions'][] = $event;
         }
 
         return true;
     }
 
     /**
      * Remove the noreply flags from attendees
      */
     public static function clear_attandee_noreply(&$event)
     {
         if (!empty($event['attendees'])) {
             foreach ((array) $event['attendees'] as $i => $attendee) {
                 unset($event['attendees'][$i]['noreply']);
             }
         }
     }
 
     /**
      * Merge certain properties from the overlay event to the base event object
      *
      * @param array The event object to be altered
      * @param array The overlay event object to be merged over $event
      * @param array List of properties not allowed to be overwritten
      */
     public static function merge_exception_data(&$event, $overlay, $blacklist = null)
     {
         $forbidden = ['id','uid','recurrence','recurrence_date','thisandfuture','organizer','_attachments'];
 
         if (is_array($blacklist)) {
             $forbidden = array_merge($forbidden, $blacklist);
         }
 
         foreach ($overlay as $prop => $value) {
             if ($prop == 'start' || $prop == 'end') {
                 // handled by merge_exception_dates() below
             }
             else if ($prop == 'thisandfuture' && $overlay['_instance'] == $event['_instance']) {
                 $event[$prop] = $value;
             }
             else if ($prop[0] != '_' && !in_array($prop, $forbidden)) {
                 $event[$prop] = $value;
             }
         }
 
         self::merge_exception_dates($event, $overlay);
     }
 
     /**
      * Merge start/end date from the overlay event to the base event object
      *
      * @param array The event object to be altered
      * @param array The overlay event object to be merged over $event
      */
     public static function merge_exception_dates(&$event, $overlay)
     {
         // compute date offset from the exception
         if ($overlay['start'] instanceof DateTimeInterface && $overlay['recurrence_date'] instanceof DateTimeInterface) {
             $date_offset = $overlay['recurrence_date']->diff($overlay['start']);
         }
 
         foreach (['start', 'end'] as $prop) {
             $value = $overlay[$prop];
             if (isset($event[$prop]) && $event[$prop] instanceof DateTimeInterface) {
                 // set date value if overlay is an exception of the current instance
                 if (substr($overlay['_instance'], 0, 8) == substr($event['_instance'], 0, 8)) {
                     $event[$prop]->setDate(intval($value->format('Y')), intval($value->format('n')), intval($value->format('j')));
                 }
                 // apply date offset
                 else if (!empty($date_offset)) {
                     $event[$prop]->add($date_offset);
                 }
                 // adjust time of the recurring event instance
                 $event[$prop]->setTime($value->format('G'), intval($value->format('i')), intval($value->format('s')));
             }
         }
     }
 
     /**
      * Get events from source.
      *
      * @param int    Event's new start (unix timestamp)
      * @param int    Event's new end (unix timestamp)
      * @param string Search query (optional)
      * @param mixed  List of calendar IDs to load events from (either as array or comma-separated string)
      * @param bool   Include virtual events (optional)
      * @param int    Only list events modified since this time (unix timestamp)
      *
      * @return array A list of event records
      */
     public function load_events($start, $end, $search = null, $calendars = null, $virtual = 1, $modifiedsince = null)
     {
         if ($calendars && is_string($calendars)) {
             $calendars = explode(',', $calendars);
         }
         else if (!$calendars) {
             $this->_read_calendars();
             $calendars = array_keys($this->calendars);
         }
 
         $query      = [];
         $events     = [];
         $categories = [];
 
         if ($modifiedsince) {
             $query[] = ['changed', '>=', $modifiedsince];
         }
 
         foreach ($calendars as $cid) {
             if ($storage = $this->get_calendar($cid)) {
                 $events = array_merge($events, $storage->list_events($start, $end, $search, $virtual, $query));
                 $categories += $storage->categories;
             }
         }
 
         // add events from the address books birthday calendar
         if (in_array(self::BIRTHDAY_CALENDAR_ID, $calendars)) {
             $events = array_merge($events, $this->load_birthday_events($start, $end, $search, $modifiedsince));
         }
 
         // add new categories to user prefs
         $old_categories = $this->rc->config->get('calendar_categories', $this->default_categories);
         $newcats = array_udiff(
             array_keys($categories),
             array_keys($old_categories),
             function($a, $b) { return strcasecmp($a, $b); }
         );
 
         if (!empty($newcats)) {
             foreach ($newcats as $category) {
                 $old_categories[$category] = '';  // no color set yet
             }
             $this->rc->user->save_prefs(['calendar_categories' => $old_categories]);
         }
 
         array_walk($events, 'kolab_driver::to_rcube_event');
 
         return $events;
     }
 
     /**
      * Get number of events in the given calendar
      *
      * @param mixed List of calendar IDs to count events (either as array or comma-separated string)
      * @param int   Date range start (unix timestamp)
      * @param int   Date range end (unix timestamp)
      *
      * @return array Hash array with counts grouped by calendar ID
      */
     public function count_events($calendars, $start, $end = null)
     {
         $counts = [];
 
         if ($calendars && is_string($calendars)) {
             $calendars = explode(',', $calendars);
         }
         else if (!$calendars) {
             $this->_read_calendars();
             $calendars = array_keys($this->calendars);
         }
 
         foreach ($calendars as $cid) {
             if ($storage = $this->get_calendar($cid)) {
                 $counts[$cid] = $storage->count_events($start, $end);
             }
         }
 
          return $counts;
     }
 
     /**
      * Get a list of pending alarms to be displayed to the user
      *
      * @see calendar_driver::pending_alarms()
      */
     public function pending_alarms($time, $calendars = null)
     {
         $interval = 300;
         $time -= $time % 60;
 
         $slot = $time;
         $slot -= $slot % $interval;
 
         $last = $time - max(60, $this->rc->config->get('refresh_interval', 0));
         $last -= $last % $interval;
 
         // only check for alerts once in 5 minutes
         if ($last == $slot) {
             return [];
         }
 
         if ($calendars && is_string($calendars)) {
             $calendars = explode(',', $calendars);
         }
 
         $time = $slot + $interval;
 
         $alarms     = [];
         $candidates = [];
         $query      = [['tags', '=', 'x-has-alarms']];
 
         $this->_read_calendars();
 
         foreach ($this->calendars as $cid => $calendar) {
             // skip calendars with alarms disabled
             if (!$calendar->alarms || ($calendars && !in_array($cid, $calendars))) {
                 continue;
             }
 
             foreach ($calendar->list_events($time, $time + 86400 * 365, null, 1, $query) as $e) {
                 // add to list if alarm is set
                 $alarm = libcalendaring::get_next_alarm($e);
                 if ($alarm && !empty($alarm['time']) && $alarm['time'] >= $last
                     && in_array($alarm['action'], $this->alarm_types)
                 ) {
                     $id = $alarm['id'];  // use alarm-id as primary identifier
                     $candidates[$id] = [
                         'id'       => $id,
                         'title'    => $e['title'],
                         'location' => $e['location'],
                         'start'    => $e['start'],
                         'end'      => $e['end'],
                         'notifyat' => $alarm['time'],
                         'action'   => $alarm['action'],
                     ];
                 }
             }
         }
 
         // get alarm information stored in local database
         if (!empty($candidates)) {
             $dbdata = [];
             $alarm_ids = array_map([$this->rc->db, 'quote'], array_keys($candidates));
 
             $result = $this->rc->db->query("SELECT *"
                 . " FROM " . $this->rc->db->table_name('kolab_alarms', true)
                 . " WHERE `alarm_id` IN (" . join(',', $alarm_ids) . ")"
                     . " AND `user_id` = ?",
                 $this->rc->user->ID
             );
 
             while ($result && ($e = $this->rc->db->fetch_assoc($result))) {
                 $dbdata[$e['alarm_id']] = $e;
             }
 
             foreach ($candidates as $id => $alarm) {
                 // skip dismissed alarms
                 if ($dbdata[$id]['dismissed']) {
                     continue;
                 }
 
                 // snooze function may have shifted alarm time
                 $notifyat = $dbdata[$id]['notifyat'] ? strtotime($dbdata[$id]['notifyat']) : $alarm['notifyat'];
                 if ($notifyat <= $time) {
                     $alarms[] = $alarm;
                 }
             }
         }
 
         return $alarms;
     }
 
     /**
      * Feedback after showing/sending an alarm notification
      *
      * @see calendar_driver::dismiss_alarm()
      */
     public function dismiss_alarm($alarm_id, $snooze = 0)
     {
         $alarms_table = $this->rc->db->table_name('kolab_alarms', true);
 
         // delete old alarm entry
         $this->rc->db->query("DELETE FROM $alarms_table"
             . " WHERE `alarm_id` = ? AND `user_id` = ?",
             $alarm_id,
             $this->rc->user->ID
         );
 
         // set new notifyat time or unset if not snoozed
         $notifyat = $snooze > 0 ? date('Y-m-d H:i:s', time() + $snooze) : null;
 
         $query = $this->rc->db->query("INSERT INTO $alarms_table"
             . " (`alarm_id`, `user_id`, `dismissed`, `notifyat`)"
             . " VALUES (?, ?, ?, ?)",
             $alarm_id,
             $this->rc->user->ID,
             $snooze > 0 ? 0 : 1,
             $notifyat
         );
 
         return $this->rc->db->affected_rows($query);
     }
 
     /**
      * List attachments from the given event
      */
     public function list_attachments($event)
     {
         if (!($storage = $this->get_calendar($event['calendar']))) {
             return false;
         }
 
         $event = $storage->get_event($event['id']);
 
         return $event['attachments'];
     }
 
     /**
      * Get attachment properties
      */
     public function get_attachment($id, $event)
     {
         if (!($storage = $this->get_calendar($event['calendar']))) {
             return false;
         }
 
         // get old revision of event
         if (!empty($event['rev'])) {
             $event = $this->get_event_revison($event, $event['rev'], true);
         }
         else {
             $event = $storage->get_event($event['id']);
         }
 
         if ($event) {
             $attachments = isset($event['_attachments']) ? $event['_attachments'] : $event['attachments'];
             foreach ((array) $attachments as $idx => $att) {
                 if ((isset($att['id']) && $att['id'] == $id) || (!isset($att['id']) && $idx == $id)) {
                     return $att;
                 }
             }
         }
     }
 
     /**
      * Get attachment body
      * @see calendar_driver::get_attachment_body()
      */
     public function get_attachment_body($id, $event)
     {
         if (!($cal = $this->get_calendar($event['calendar']))) {
             return false;
         }
 
         // get old revision of event
         if (!empty($event['rev'])) {
             if (empty($this->bonnie_api)) {
                 return false;
             }
 
             $cid = substr($id, 4);
 
             // call Bonnie API and get the raw mime message
             list($uid, $mailbox, $msguid) = $this->_resolve_event_identity($event);
             if ($msg_raw = $this->bonnie_api->rawdata('event', $uid, $event['rev'], $mailbox, $msguid)) {
                 // parse the message and find the part with the matching content-id
                 $message = rcube_mime::parse_message($msg_raw);
                 foreach ((array) $message->parts as $part) {
                     if (!empty($part->headers['content-id']) && trim($part->headers['content-id'], '<>') == $cid) {
                         return $part->body;
                     }
                 }
             }
 
             return false;
         }
 
         return $cal->get_attachment_body($id, $event);
     }
 
     /**
      * Build a struct representing the given message reference
      *
      * @see calendar_driver::get_message_reference()
      */
     public function get_message_reference($uri_or_headers, $folder = null)
     {
         if (is_object($uri_or_headers)) {
             $uri_or_headers = kolab_storage_config::get_message_uri($uri_or_headers, $folder);
         }
 
         if (is_string($uri_or_headers)) {
             return kolab_storage_config::get_message_reference($uri_or_headers, 'event');
         }
 
         return false;
     }
 
     /**
      * List availabale categories
      * The default implementation reads them from config/user prefs
      */
     public function list_categories()
     {
         // FIXME: complete list with categories saved in config objects (KEP:12)
         return $this->rc->config->get('calendar_categories', $this->default_categories);
     }
 
     /**
      * Create instances of a recurring event
      *
      * @param array    Hash array with event properties
      * @param DateTime Start date of the recurrence window
      * @param DateTime End date of the recurrence window
      *
      * @return array List of recurring event instances
      */
     public function get_recurring_events($event, $start, $end = null)
     {
         // load the given event data into a libkolabxml container
         if (empty($event['_formatobj'])) {
             $event_xml = new kolab_format_event();
             $event_xml->set($event);
             $event['_formatobj'] = $event_xml;
         }
 
         $this->_read_calendars();
         $storage = reset($this->calendars);
 
         return $storage->get_recurring_events($event, $start, $end);
     }
 
     /**
      *
      */
     protected function get_recurrence_count($event, $dtstart)
     {
         // load the given event data into a libkolabxml container
         if (empty($event['_formatobj'])) {
             $event_xml = new kolab_format_event();
             $event_xml->set($event);
             $event['_formatobj'] = $event_xml;
         }
 
         // use libkolab to compute recurring events
         $recurrence = new kolab_date_recurrence($event['_formatobj']);
 
         $count = 0;
         while (($next_event = $recurrence->next_instance()) && $next_event['start'] <= $dtstart && $count < 1000) {
             $count++;
         }
 
         return $count;
     }
 
     /**
      * Fetch free/busy information from a person within the given range
      */
     public function get_freebusy_list($email, $start, $end)
     {
         if (empty($email)/* || $end < time()*/) {
             return false;
         }
 
         // map vcalendar fbtypes to internal values
         $fbtypemap = [
             'FREE'            => calendar::FREEBUSY_FREE,
             'BUSY-TENTATIVE'  => calendar::FREEBUSY_TENTATIVE,
             'X-OUT-OF-OFFICE' => calendar::FREEBUSY_OOF,
             'OOF'             => calendar::FREEBUSY_OOF
         ];
 
         // ask kolab server first
         try {
             $request_config = [
                 'store_body'       => true,
                 'follow_redirects' => true,
             ];
 
             $request  = libkolab::http_request($this->storage->get_freebusy_url($email), 'GET', $request_config);
             $response = $request->send();
 
             // authentication required
             if ($response->getStatus() == 401) {
                 $request->setAuth($this->rc->user->get_username(), $this->rc->decrypt($_SESSION['password']));
                 $response = $request->send();
             }
 
             if ($response->getStatus() == 200) {
                 $fbdata = $response->getBody();
             }
 
             unset($request, $response);
         }
         catch (Exception $e) {
             PEAR::raiseError("Error fetching free/busy information: " . $e->getMessage());
         }
 
         // get free-busy url from contacts
         if (empty($fbdata)) {
             $fburl = null;
             foreach ((array) $this->rc->config->get('autocomplete_addressbooks', 'sql') as $book) {
                 $abook = $this->rc->get_address_book($book);
 
                 if ($result = $abook->search(['email'], $email, true, true, true/*, 'freebusyurl'*/)) {
                     while ($contact = $result->iterate()) {
                         if (!empty($contact['freebusyurl'])) {
                             $fbdata = @file_get_contents($contact['freebusyurl']);
                             break;
                         }
                     }
                 }
 
                 if (!empty($fbdata)) {
                     break;
                 }
             }
         }
 
         // parse free-busy information using Horde classes
         if (!empty($fbdata)) {
             $ical = $this->cal->get_ical();
             $ical->import($fbdata);
             if ($fb = $ical->freebusy) {
                 $result = [];
                 foreach ($fb['periods'] as $tuple) {
                     list($from, $to, $type) = $tuple;
                     $result[] = [
                         $from->format('U'),
                         $to->format('U'),
                         isset($fbtypemap[$type]) ? $fbtypemap[$type] : calendar::FREEBUSY_BUSY
                     ];
                 }
 
                 // we take 'dummy' free-busy lists as "unknown"
                 if (empty($result) && !empty($fb['comment']) && stripos($fb['comment'], 'dummy')) {
                     return false;
                 }
 
                 // set period from $start till the begin of the free-busy information as 'unknown'
                 if (!empty($fb['start']) && ($fbstart = $fb['start']->format('U')) && $start < $fbstart) {
                     array_unshift($result, [$start, $fbstart, calendar::FREEBUSY_UNKNOWN]);
                 }
                 // pad period till $end with status 'unknown'
                 if (!empty($fb['end']) && ($fbend = $fb['end']->format('U')) && $fbend < $end) {
                     $result[] = [$fbend, $end, calendar::FREEBUSY_UNKNOWN];
                 }
 
                 return $result;
             }
         }
 
         return false;
     }
 
     /**
      * Handler to push folder triggers when sent from client.
      * Used to push free-busy changes asynchronously after updating an event
      */
     public function push_freebusy()
     {
         // make shure triggering completes
         set_time_limit(0);
         ignore_user_abort(true);
 
         $cal = rcube_utils::get_input_value('source', rcube_utils::INPUT_GPC);
         if (!($cal = $this->get_calendar($cal))) {
             return false;
         }
 
         // trigger updates on folder
         $trigger = $cal->storage->trigger();
         if (is_object($trigger) && is_a($trigger, 'PEAR_Error')) {
             rcube::raise_error([
                     'code' => 900, 'file' => __FILE__, 'line' => __LINE__,
                     'message' => "Failed triggering folder. Error was " . $trigger->getMessage()
                 ],
                 true, false
             );
         }
 
         exit;
     }
 
     /**
      * Convert from driver format to external caledar app data
      */
     public static function to_rcube_event(&$record)
     {
         if (!is_array($record)) {
             return $record;
         }
 
-        $record['id'] = $record['uid'];
+        $record['id'] = $record['uid'] ?? null;
 
         if (!empty($record['_instance'])) {
             $record['id'] .= '-' . $record['_instance'];
 
             if (empty($record['recurrence_id']) && !empty($record['recurrence'])) {
                 $record['recurrence_id'] = $record['uid'];
             }
         }
 
         // all-day events go from 12:00 - 13:00
         if ($record['start'] instanceof DateTimeInterface && $record['end'] <= $record['start'] && !empty($record['allday'])) {
             $record['end'] = clone $record['start'];
             $record['end']->add(new DateInterval('PT1H'));
         }
 
         // translate internal '_attachments' to external 'attachments' list
         if (!empty($record['_attachments'])) {
             $attachments = [];
 
             foreach ($record['_attachments'] as $key => $attachment) {
                 if ($attachment !== false) {
                     if (empty($attachment['name'])) {
                         $attachment['name'] = $key;
                     }
 
                     unset($attachment['path'], $attachment['content']);
                     $attachments[] = $attachment;
                 }
             }
 
             $record['attachments'] = $attachments;
         }
 
         if (!empty($record['attendees'])) {
             foreach ((array) $record['attendees'] as $i => $attendee) {
                 if (isset($attendee['delegated-from']) && is_array($attendee['delegated-from'])) {
                     $record['attendees'][$i]['delegated-from'] = join(', ', $attendee['delegated-from']);
                 }
                 if (isset($attendee['delegated-to']) && is_array($attendee['delegated-to'])) {
                     $record['attendees'][$i]['delegated-to'] = join(', ', $attendee['delegated-to']);
                 }
             }
         }
 
         // Roundcube only supports one category assignment
         if (!empty($record['categories']) && is_array($record['categories'])) {
             $record['categories'] = $record['categories'][0];
         }
 
         // the cancelled flag transltes into status=CANCELLED
         if (!empty($record['cancelled'])) {
             $record['status'] = 'CANCELLED';
         }
 
         // The web client only supports DISPLAY type of alarms
         if (!empty($record['alarms'])) {
             $record['alarms'] = preg_replace('/:[A-Z]+$/', ':DISPLAY', $record['alarms']);
         }
 
         // remove empty recurrence array
         if (empty($record['recurrence'])) {
             unset($record['recurrence']);
         }
         // clean up exception data
         else if (!empty($record['recurrence']['EXCEPTIONS'])) {
             array_walk($record['recurrence']['EXCEPTIONS'], function(&$exception) {
                 unset($exception['_mailbox'], $exception['_msguid'],
                     $exception['_formatobj'], $exception['_attachments']
                 );
             });
         }
 
         unset($record['_mailbox'], $record['_msguid'], $record['_type'], $record['_size'],
             $record['_formatobj'], $record['_attachments'], $record['exceptions'], $record['x-custom']
         );
 
         return $record;
     }
 
     /**
      *
      */
     public static function from_rcube_event($event, $old = [])
     {
         kolab_format::merge_attachments($event, $old);
 
         return $event;
     }
 
 
     /**
      * Set CSS class according to the event's attendde partstat
      */
     public static function add_partstat_class($event, $partstats, $user = null)
     {
         // set classes according to PARTSTAT
         if (!empty($event['attendees'])) {
             $user_emails = libcalendaring::get_instance()->get_user_emails($user);
             $partstat = 'UNKNOWN';
 
             foreach ($event['attendees'] as $attendee) {
                 if (in_array($attendee['email'], $user_emails)) {
                     if (!empty($attendee['status'])) {
                         $partstat = $attendee['status'];
                     }
                     break;
                 }
             }
 
             if (in_array($partstat, $partstats)) {
                 $event['className'] = trim($event['className'] . ' fc-invitation-' . strtolower($partstat));
             }
         }
 
         return $event;
     }
 
     /**
      * Provide a list of revisions for the given event
      *
      * @param array $event Hash array with event properties
      *
      * @return array List of changes, each as a hash array
      * @see calendar_driver::get_event_changelog()
      */
     public function get_event_changelog($event)
     {
         if (empty($this->bonnie_api)) {
             return false;
         }
 
         list($uid, $mailbox, $msguid) = $this->_resolve_event_identity($event);
 
         $result = $this->bonnie_api->changelog('event', $uid, $mailbox, $msguid);
         if (is_array($result) && $result['uid'] == $uid) {
             return $result['changes'];
         }
 
         return false;
     }
 
     /**
      * Get a list of property changes beteen two revisions of an event
      *
      * @param array $event Hash array with event properties
      * @param mixed $rev1  Old Revision
      * @param mixed $rev2  New Revision
      *
      * @return array List of property changes, each as a hash array
      * @see calendar_driver::get_event_diff()
      */
     public function get_event_diff($event, $rev1, $rev2)
     {
         if (empty($this->bonnie_api)) {
             return false;
         }
 
         list($uid, $mailbox, $msguid) = $this->_resolve_event_identity($event);
 
         // get diff for the requested recurrence instance
         $instance_id = $event['id'] != $uid ? substr($event['id'], strlen($uid) + 1) : null;
 
         // call Bonnie API
         $result = $this->bonnie_api->diff('event', $uid, $rev1, $rev2, $mailbox, $msguid, $instance_id);
 
         if (is_array($result) && $result['uid'] == $uid) {
             $result['rev1'] = $rev1;
             $result['rev2'] = $rev2;
 
             $keymap = [
                 'dtstart'  => 'start',
                 'dtend'    => 'end',
                 'dstamp'   => 'changed',
                 'summary'  => 'title',
                 'alarm'    => 'alarms',
                 'attendee' => 'attendees',
                 'attach'   => 'attachments',
                 'rrule'    => 'recurrence',
                 'transparency' => 'free_busy',
                 'lastmodified-date' => 'changed',
             ];
 
             $prop_keymaps = [
                 'attachments' => ['fmttype' => 'mimetype', 'label' => 'name'],
                 'attendees'   => ['partstat' => 'status'],
             ];
 
             $special_changes = [];
 
             // map kolab event properties to keys the client expects
             array_walk($result['changes'], function(&$change, $i) use ($keymap, $prop_keymaps, $special_changes) {
                 if (array_key_exists($change['property'], $keymap)) {
                     $change['property'] = $keymap[$change['property']];
                 }
                 // translate free_busy values
                 if ($change['property'] == 'free_busy') {
                     $change['old'] = !empty($old['old']) ? 'free' : 'busy';
                     $change['new'] = !empty($old['new']) ? 'free' : 'busy';
                 }
 
                 // map alarms trigger value
                 if ($change['property'] == 'alarms') {
                     if (!empty($change['old']['trigger'])) {
                         $change['old']['trigger'] = $change['old']['trigger']['value'];
                     }
                     if (!empty($change['new']['trigger'])) {
                         $change['new']['trigger'] = $change['new']['trigger']['value'];
                     }
                 }
 
                 // make all property keys uppercase
                 if ($change['property'] == 'recurrence') {
                     $special_changes['recurrence'] = $i;
                     foreach (['old', 'new'] as $m) {
                         if (!empty($change[$m])) {
                             $props = [];
                             foreach ($change[$m] as $k => $v) {
                                 $props[strtoupper($k)] = $v;
                             }
                             $change[$m] = $props;
                         }
                     }
                 }
 
                 // map property keys names
                 if (!empty($prop_keymaps[$change['property']])) {
                     foreach ($prop_keymaps[$change['property']] as $k => $dest) {
                         if (!empty($change['old']) && array_key_exists($k, $change['old'])) {
                             $change['old'][$dest] = $change['old'][$k];
                             unset($change['old'][$k]);
                         }
                         if (!empty($change['new']) && array_key_exists($k, $change['new'])) {
                             $change['new'][$dest] = $change['new'][$k];
                             unset($change['new'][$k]);
                         }
                     }
                 }
 
                 if ($change['property'] == 'exdate') {
                     $special_changes['exdate'] = $i;
                 }
                 else if ($change['property'] == 'rdate') {
                     $special_changes['rdate'] = $i;
                 }
             });
 
             // merge some recurrence changes
             foreach (['exdate', 'rdate'] as $prop) {
                 if (array_key_exists($prop, $special_changes)) {
                     $exdate = $result['changes'][$special_changes[$prop]];
                     if (array_key_exists('recurrence', $special_changes)) {
                         $recurrence = &$result['changes'][$special_changes['recurrence']];
                     }
                     else {
                         $i = count($result['changes']);
                         $result['changes'][$i] = ['property' => 'recurrence', 'old' => [], 'new' => []];
                         $recurrence = &$result['changes'][$i]['recurrence'];
                     }
                     $key = strtoupper($prop);
                     $recurrence['old'][$key] = $exdate['old'];
                     $recurrence['new'][$key] = $exdate['new'];
                     unset($result['changes'][$special_changes[$prop]]);
                 }
             }
 
             return $result;
         }
 
         return false;
     }
 
     /**
      * Return full data of a specific revision of an event
      *
      * @param array Hash array with event properties
      * @param mixed $rev Revision number
      *
      * @return array Event object as hash array
      * @see calendar_driver::get_event_revison()
      */
     public function get_event_revison($event, $rev, $internal = false)
     {
         if (empty($this->bonnie_api)) {
             return false;
         }
 
         $eventid = $event['id'];
         $calid   = $event['calendar'];
 
         list($uid, $mailbox, $msguid) = $this->_resolve_event_identity($event);
 
         // call Bonnie API
         $result = $this->bonnie_api->get('event', $uid, $rev, $mailbox, $msguid);
         if (is_array($result) && $result['uid'] == $uid && !empty($result['xml'])) {
             $format = kolab_format::factory('event');
             $format->load($result['xml']);
             $event = $format->to_array();
             $format->get_attachments($event, true);
 
             // get the right instance from a recurring event
             if ($eventid != $event['uid']) {
                 $instance_id = substr($eventid, strlen($event['uid']) + 1);
 
                 // check for recurrence exception first
                 if ($instance = $format->get_instance($instance_id)) {
                     $event = $instance;
                 }
                 else {
                     // not a exception, compute recurrence...
                     $event['_formatobj'] = $format;
                     $recurrence_date = rcube_utils::anytodatetime($instance_id, $event['start']->getTimezone());
                     foreach ($this->get_recurring_events($event, $event['start'], $recurrence_date) as $instance) {
                         if ($instance['id'] == $eventid) {
                             $event = $instance;
                             break;
                         }
                     }
                 }
             }
 
             if ($format->is_valid()) {
                 $event['calendar'] = $calid;
                 $event['rev']      = $result['rev'];
 
                 return $internal ? $event : self::to_rcube_event($event);
             }
         }
 
         return false;
     }
 
     /**
      * Command the backend to restore a certain revision of an event.
      * This shall replace the current event with an older version.
      *
      * @param mixed $event UID string or hash array with event properties:
      *              id: Event identifier
      *        calendar: Calendar identifier
      * @param mixed $rev   Revision number
      *
      * @return bool True on success, False on failure
      */
     public function restore_event_revision($event, $rev)
     {
         if (empty($this->bonnie_api)) {
             return false;
         }
 
         list($uid, $mailbox, $msguid) = $this->_resolve_event_identity($event);
 
         $calendar = $this->get_calendar($event['calendar']);
         $success  = false;
 
         if ($calendar && $calendar->storage && $calendar->editable) {
             if ($raw_msg = $this->bonnie_api->rawdata('event', $uid, $rev, $mailbox)) {
                 $imap = $this->rc->get_storage();
 
                 // insert $raw_msg as new message
                 if ($imap->save_message($calendar->storage->name, $raw_msg, null, false)) {
                     $success = true;
 
                     // delete old revision from imap and cache
                     $imap->delete_message($msguid, $calendar->storage->name);
                     $calendar->storage->cache->set($msguid, false);
                 }
             }
         }
 
         return $success;
     }
 
     /**
      * Helper method to resolved the given event identifier into uid and folder
      *
      * @return array (uid,folder,msguid) tuple
      */
     protected function _resolve_event_identity($event)
     {
         $mailbox = $msguid = null;
 
         if (is_array($event)) {
             $uid = !empty($event['uid']) ? $event['uid'] : $event['id'];
 
             if (($cal = $this->get_calendar($event['calendar'])) && !($cal instanceof kolab_invitation_calendar)) {
                 $mailbox = $cal->get_mailbox_id();
 
                 // get event object from storage in order to get the real object uid an msguid
                 if ($ev = $cal->get_event($event['id'])) {
                     $msguid = $ev['_msguid'];
                     $uid = $ev['uid'];
                 }
             }
         }
         else {
             $uid = $event;
 
             // get event object from storage in order to get the real object uid an msguid
             if ($ev = $this->get_event($event)) {
                 $mailbox = $ev['_mailbox'];
                 $msguid  = $ev['_msguid'];
                 $uid     = $ev['uid'];
             }
         }
 
         return array($uid, $mailbox, $msguid);
     }
 
     /**
      * Callback function to produce driver-specific calendar create/edit form
      *
      * @param string Request action 'form-edit|form-new'
      * @param array  Calendar properties (e.g. id, color)
      * @param array  Edit form fields
      *
      * @return string HTML content of the form
      */
     public function calendar_form($action, $calendar, $formfields)
     {
         $special_calendars = [
             self::BIRTHDAY_CALENDAR_ID,
             self::INVITATIONS_CALENDAR_PENDING,
             self::INVITATIONS_CALENDAR_DECLINED
         ];
 
         // show default dialog for birthday calendar
         if (in_array($calendar['id'], $special_calendars)) {
             if ($calendar['id'] != self::BIRTHDAY_CALENDAR_ID) {
                 unset($formfields['showalarms']);
             }
 
             // General tab
             $form['props'] = [
                 'name'   => $this->rc->gettext('properties'),
                 'fields' => $formfields,
             ];
 
             return kolab_utils::folder_form($form, '', 'calendar');
         }
 
         $this->_read_calendars();
 
         if (!empty($calendar['id']) && ($cal = $this->calendars[$calendar['id']])) {
             $folder = $cal->get_realname(); // UTF7
             $color  = $cal->get_color();
         }
         else {
             $folder = '';
             $color  = '';
         }
 
         $hidden_fields[] = ['name' => 'oldname', 'value' => $folder];
 
         $storage = $this->rc->get_storage();
         $delim   = $storage->get_hierarchy_delimiter();
         $form    = [];
 
         if (strlen($folder)) {
             $path_imap = explode($delim, $folder);
             array_pop($path_imap);  // pop off name part
             $path_imap = implode($delim, $path_imap);
 
             $options = $storage->folder_info($folder);
         }
         else {
             $path_imap = '';
         }
 
         // General tab
         $form['props'] = [
             'name'   => $this->rc->gettext('properties'),
             'fields' => [],
         ];
 
         $protected = !empty($options) && (!empty($options['norename']) || !empty($options['protected']));
         // Disable folder name input
         if ($protected) {
             $input_name = new html_hiddenfield(['name' => 'name', 'id' => 'calendar-name']);
             $formfields['name']['value'] = $this->storage->object_name($folder)
                 . $input_name->show($folder);
         }
 
         // calendar name (default field)
         $form['props']['fields']['location'] = $formfields['name'];
 
         if ($protected) {
             // prevent user from moving folder
             $hidden_fields[] = ['name' => 'parent', 'value' => $path_imap];
         }
         else {
             $select = $this->storage->folder_selector('event', ['name' => 'parent', 'id' => 'calendar-parent'], $folder);
 
             $form['props']['fields']['path'] = [
                 'id'    => 'calendar-parent',
                 'label' => $this->cal->gettext('parentcalendar'),
                 'value' => $select->show(strlen($folder) ? $path_imap : ''),
             ];
         }
 
         // calendar color (default field)
         $form['props']['fields']['color']  = $formfields['color'];
         $form['props']['fields']['alarms'] = $formfields['showalarms'];
 
         return kolab_utils::folder_form($form, $folder, 'calendar', $hidden_fields);
     }
 
     /**
      * Handler for user_delete plugin hook
      */
     public function user_delete($args)
     {
         $db = $this->rc->get_dbh();
         foreach (['kolab_alarms', 'itipinvitations'] as $table) {
             $db->query("DELETE FROM " . $this->rc->db->table_name($table, true)
                 . " WHERE `user_id` = ?", $args['user']->ID);
         }
     }
 }
diff --git a/plugins/libcalendaring/lib/libcalendaring_recurrence.php b/plugins/libcalendaring/lib/libcalendaring_recurrence.php
index df802b7b..99e05679 100644
--- a/plugins/libcalendaring/lib/libcalendaring_recurrence.php
+++ b/plugins/libcalendaring/lib/libcalendaring_recurrence.php
@@ -1,277 +1,294 @@
 <?php
 
 use Sabre\VObject\Recur\EventIterator;
 
 /**
  * Recurrence computation class for shared use.
  *
  * Uitility class to compute recurrence dates from the given rules.
  *
  * @author Aleksander Machniak <machniak@apheleia-it.ch
  *
  * Copyright (C) 2012-2022, Apheleia IT AG <contact@apheleia-it.ch>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
  * published by the Free Software Foundation, either version 3 of the
  * License, or (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU Affero General Public License for more details.
  *
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 class libcalendaring_recurrence
 {
     protected $lib;
     protected $start;
     protected $engine;
     protected $recurrence;
     protected $dateonly = false;
     protected $event;
     protected $duration;
+    protected $isStart = true;
 
     /**
      * Default constructor
      *
      * @param libcalendaring $lib   The libcalendaring plugin instance
      * @param array          $event The event object to operate on
      */
     function __construct($lib, $event = null)
     {
         $this->lib   = $lib;
         $this->event = $event;
 
         if (!empty($event)) {
             if (!empty($event['start']) && is_object($event['start'])
                 && !empty($event['end']) && is_object($event['end'])
             ) {
                 $this->duration = $event['start']->diff($event['end']);
             }
 
             $this->init($event['recurrence'], $event['start']);
         }
     }
 
     /**
      * Initialize recurrence engine
      *
      * @param array    The recurrence properties
      * @param DateTime The recurrence start date
      */
     public function init($recurrence, $start)
     {
         $this->start      = $start;
         $this->dateonly   = !empty($start->_dateonly) || !empty($this->event['allday']);
         $this->recurrence = $recurrence;
 
         $event = [
             'uid' => '1',
             'allday' => $this->dateonly,
             'recurrence' => $recurrence,
             'start' => $start,
             // TODO: END/DURATION ???
             // TODO: moved occurrences ???
         ];
 
         $vcalendar = new libcalendaring_vcalendar($this->lib->timezone);
 
         $ve = $vcalendar->toSabreComponent($event);
 
         $this->engine = new EventIterator($ve, null, $this->lib->timezone);
     }
 
     /**
      * Get date/time of the next occurence of this event, and push the iterator.
      *
      * @return DateTime|false object or False if recurrence ended
      */
     public function next_start()
     {
         try {
             $this->engine->next();
             $current = $this->engine->getDtStart();
         }
         catch (Exception $e) {
             // do nothing
         }
 
         return $current ? $this->toDateTime($current) : false;
     }
 
     /**
      * Get the next recurring instance of this event
      *
      * @return array|false Array with event properties or False if recurrence ended
      */
     public function next_instance()
     {
-        if ($next_start = $this->next_start()) {
+        // Here's the workaround for an issue for an event with its start date excluded
+        // E.g. A daily event starting on 10th which is one of EXDATE dates
+        // should return 11th as next_instance() when called for the first time.
+        // Looks like Sabre is setting internal "current date" to 11th on such an object
+        // initialization, therefore calling next() would move it to 12th.
+        if ($this->isStart && ($next_start = $this->engine->getDtStart())
+            && $next_start->format('Ymd') != $this->start->format('Ymd')
+        ) {
+            $next_start = $this->toDateTime($next_start);
+        }
+        else {
+            $next_start = $this->next_start();
+        }
+
+        $this->isStart = false;
+
+        if ($next_start) {
             $next = $this->event;
             $next['start'] = $next_start;
 
             if ($this->duration) {
                 $next['end'] = clone $next_start;
                 $next['end']->add($this->duration);
             }
 
             $next['recurrence_date'] = clone $next_start;
-            $next['_instance'] = libcalendaring::recurrence_instance_identifier($next, !empty($this->event['allday']));
+            $next['_instance'] = libcalendaring::recurrence_instance_identifier($next);
 
             unset($next['_formatobj']);
 
             return $next;
         }
 
         return false;
     }
 
     /**
      * Get the date of the end of the last occurrence of this recurrence cycle
      *
      * @return DateTime|false End datetime of the last occurrence or False if there's no end date
      */
     public function end()
     {
         // recurrence end date is given
         if (isset($this->recurrence['UNTIL']) && $this->recurrence['UNTIL'] instanceof DateTimeInterface) {
             return $this->recurrence['UNTIL'];
         }
 
         // Run through all items till we reach the end, or limit of iterations
         // Note: Sabre has a limits of iteration in VObject\Settings, so it is not an infinite loop
         try {
             foreach ($this->engine as $end) {
                 // do nothing
             }
         }
         catch (Exception $e) {
             // do nothing
         }
 /*
         if (empty($end) && isset($this->event['start']) && $this->event['start'] instanceof DateTimeInterface) {
             // determine a reasonable end date if none given
             $end = clone $this->event['start'];
             $end->add(new DateInterval('P100Y'));
         }
 */
 
         return isset($end) ? $this->toDateTime($end) : false;
     }
 
     /**
      * Find date/time of the first occurrence (excluding start date)
      *
      * @return DateTime|null First occurrence
      */
     public function first_occurrence()
     {
         $start    = clone $this->start;
         $interval = $this->recurrence['INTERVAL'] ?? 1;
         $freq     = $this->recurrence['FREQ'] ?? null;
 
         switch ($freq) {
         case 'WEEKLY':
             if (empty($this->recurrence['BYDAY'])) {
                 return $start;
             }
 
             $start->sub(new DateInterval("P{$interval}W"));
             break;
 
         case 'MONTHLY':
             if (empty($this->recurrence['BYDAY']) && empty($this->recurrence['BYMONTHDAY'])) {
                 return $start;
             }
 
             $start->sub(new DateInterval("P{$interval}M"));
             break;
 
         case 'YEARLY':
             if (empty($this->recurrence['BYDAY']) && empty($this->recurrence['BYMONTH'])) {
                 return $start;
             }
 
             $start->sub(new DateInterval("P{$interval}Y"));
             break;
 
         case 'DAILY':
             if (!empty($this->recurrence['BYMONTH'])) {
                 break;
             }
 
         default:
             return $start;
         }
 
         $recurrence = $this->recurrence;
 
         if (!empty($recurrence['COUNT'])) {
             // Increase count so we do not stop the loop to early
             $recurrence['COUNT'] += 100;
         }
 
         // Create recurrence that starts in the past
         $self = new self($this->lib);
         $self->init($recurrence, $start);
 
         // TODO: This method does not work the same way as the kolab_date_recurrence based on
         //       kolabcalendaring. I.e. if an event start date does not match the recurrence rule
         //       it will be returned, kolab_date_recurrence will return the next occurrence in such a case
         //       which is the intended result of this function.
         //       See some commented out test cases in tests/RecurrenceTest.php
 
         // find the first occurrence
         $found = false;
         while ($next = $self->next_start()) {
             $start = $next;
             if ($next >= $this->start) {
                 $found = true;
                 break;
             }
         }
 
         if (!$found) {
             rcube::raise_error(
                 [
                     'file' => __FILE__, 'line' => __LINE__,
                     'message' => sprintf("Failed to find a first occurrence. Start: %s, Recurrence: %s",
                         $this->start->format(DateTime::ISO8601), json_encode($recurrence)),
                 ],
                 true
             );
 
             return null;
         }
 
         return $this->toDateTime($start);
     }
 
     /**
      * Convert any DateTime into libcalendaring_datetime
      */
     protected function toDateTime($date, $useStart = true)
     {
         if ($date instanceof DateTimeInterface) {
             $date = libcalendaring_datetime::createFromFormat(
                 'Y-m-d\\TH:i:s',
                 $date->format('Y-m-d\\TH:i:s'),
                 // Sabre will loose timezone on all-day events, use the event start's timezone
                 $this->start->getTimezone()
             );
         }
 
         $date->_dateonly = $this->dateonly;
 
         if ($useStart && $this->dateonly) {
             // Sabre sets time to 00:00:00 for all-day events,
             // let's copy the time from the event's start
             $date->setTime((int) $this->start->format('H'), (int) $this->start->format('i'), (int) $this->start->format('s'));
         }
 
         return $date;
     }
 }
diff --git a/plugins/libcalendaring/libcalendaring.php b/plugins/libcalendaring/libcalendaring.php
index a7f63d1d..dbb0066c 100644
--- a/plugins/libcalendaring/libcalendaring.php
+++ b/plugins/libcalendaring/libcalendaring.php
@@ -1,1584 +1,1605 @@
 <?php
 
 /**
  * Library providing common functions for calendaring plugins
  *
  * Provides utility functions for calendar-related modules such as
  * - alarms display and dismissal
  * - attachment handling
  * - recurrence computation and UI elements
  * - ical parsing and exporting
  * - itip scheduling protocol
  *
  * @version @package_version@
  * @author Thomas Bruederli <bruederli@kolabsys.com>
  *
  * Copyright (C) 2012-2015, Kolab Systems AG <contact@kolabsys.com>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
  * published by the Free Software Foundation, either version 3 of the
  * License, or (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU Affero General Public License for more details.
  *
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
 class libcalendaring extends rcube_plugin
 {
     public $rc;
     public $timezone;
     public $gmt_offset;
     public $dst_active;
     public $timezone_offset;
     public $ical_parts = [];
     public $ical_message;
 
     public $defaults = array(
         'calendar_date_format'  => "Y-m-d",
         'calendar_date_short'   => "M-j",
         'calendar_date_long'    => "F j Y",
         'calendar_date_agenda'  => "l M-d",
         'calendar_time_format'  => "H:m",
         'calendar_first_day'    => 1,
         'calendar_first_hour'   => 6,
         'calendar_date_format_sets' => array(
             'Y-m-d' => array('d M Y',   'm-d',  'l m-d'),
             'Y/m/d' => array('d M Y',   'm/d',  'l m/d'),
             'Y.m.d' => array('d M Y',   'm.d',  'l m.d'),
             'd-m-Y' => array('d M Y',   'd-m',  'l d-m'),
             'd/m/Y' => array('d M Y',   'd/m',  'l d/m'),
             'd.m.Y' => array('d M Y',  'd.m',  'l d.m'),
             'j.n.Y' => array('d M Y',  'd.m',  'l d.m'),
             'm/d/Y' => array('M d Y',   'm/d',  'l m/d'),
         ),
     );
 
     private static $instance;
 
     private $mail_ical_parser;
 
     /**
      * Singleton getter to allow direct access from other plugins
      */
     public static function get_instance()
     {
         if (!self::$instance) {
             self::$instance = new libcalendaring(rcube::get_instance()->plugins);
             self::$instance->init_instance();
         }
 
         return self::$instance;
     }
 
     /**
      * Initializes class properties
      */
     public function init_instance()
     {
         $this->rc = rcube::get_instance();
 
         // set user's timezone
         try {
             $this->timezone = new DateTimeZone($this->rc->config->get('timezone', 'GMT'));
         }
         catch (Exception $e) {
             $this->timezone = new DateTimeZone('GMT');
         }
 
         $now = new DateTime('now', $this->timezone);
 
         $this->gmt_offset      = $now->getOffset();
         $this->dst_active      = $now->format('I');
         $this->timezone_offset = $this->gmt_offset / 3600 - $this->dst_active;
 
         $this->add_texts('localization/', false);
     }
 
     /**
      * Required plugin startup method
      */
     public function init()
     {
         // extend include path to load bundled lib classes
         $include_path = $this->home . '/lib' . PATH_SEPARATOR . ini_get('include_path');
         set_include_path($include_path);
 
         self::$instance = $this;
 
         $this->rc = rcube::get_instance();
         $this->init_instance();
 
         // include client scripts and styles
         if ($this->rc->output) {
             // add hook to display alarms
             $this->add_hook('refresh', array($this, 'refresh'));
             $this->register_action('plugin.alarms', array($this, 'alarms_action'));
             $this->register_action('plugin.expand_attendee_group', array($this, 'expand_attendee_group'));
         }
 
         // proceed initialization in startup hook
         $this->add_hook('startup', array($this, 'startup'));
     }
 
     /**
      * Startup hook
      */
     public function startup($args)
     {
         if ($this->rc->output && $this->rc->output->type == 'html') {
             $this->rc->output->set_env('libcal_settings', $this->load_settings());
             $this->include_script('libcalendaring.js');
             $this->include_stylesheet($this->local_skin_path() . '/libcal.css');
 
             $this->add_label(
                 'itipaccepted', 'itiptentative', 'itipdeclined',
                 'itipdelegated', 'expandattendeegroup', 'expandattendeegroupnodata',
                 'statusorganizer', 'statusaccepted', 'statusdeclined',
                 'statusdelegated', 'statusunknown', 'statusneeds-action',
                 'statustentative', 'statuscompleted', 'statusin-process',
                 'delegatedto', 'delegatedfrom', 'showmore'
             );
         }
 
         if ($args['task'] == 'mail') {
             if ($args['action'] == 'show' || $args['action'] == 'preview') {
                 $this->add_hook('message_load', array($this, 'mail_message_load'));
             }
         }
     }
 
     /**
      * Load iCalendar functions
      */
     public static function get_ical()
     {
         $self = self::get_instance();
         return new libcalendaring_vcalendar();
     }
 
     /**
      * Load iTip functions
      */
     public static function get_itip($domain = 'libcalendaring')
     {
         $self = self::get_instance();
         return new libcalendaring_itip($self, $domain);
     }
 
     /**
      * Load recurrence computation engine
      */
     public static function get_recurrence($object = null)
     {
         $self = self::get_instance();
         return new libcalendaring_recurrence($self, $object);
     }
 
     /**
      * Shift dates into user's current timezone
      *
      * @param mixed Any kind of a date representation (DateTime object, string or unix timestamp)
      *
      * @return object DateTime object in user's timezone
      */
     public function adjust_timezone($dt, $dateonly = false)
     {
         if (is_numeric($dt)) {
             $dt = new DateTime('@'.$dt);
         }
         else if (is_string($dt)) {
             $dt = rcube_utils::anytodatetime($dt);
         }
 
         if ($dt instanceof DateTimeInterface && empty($dt->_dateonly) && !$dateonly) {
             $dt = $dt->setTimezone($this->timezone);
         }
 
         return $dt;
     }
 
     /**
      *
      */
     public function load_settings()
     {
         $this->date_format_defaults();
 
         $settings = array();
         $keys     = array('date_format', 'time_format', 'date_short', 'date_long', 'date_agenda');
 
         foreach ($keys as $key) {
             $settings[$key] = (string)$this->rc->config->get('calendar_' . $key, $this->defaults['calendar_' . $key]);
             $settings[$key] = self::from_php_date_format($settings[$key]);
         }
 
         $settings['dates_long']  = $settings['date_long'];
         $settings['first_day']   = (int)$this->rc->config->get('calendar_first_day', $this->defaults['calendar_first_day']);
         $settings['timezone']    = $this->timezone_offset;
         $settings['dst']         = $this->dst_active;
 
         // localization
         $settings['days'] = array(
             $this->rc->gettext('sunday'),   $this->rc->gettext('monday'),
             $this->rc->gettext('tuesday'),  $this->rc->gettext('wednesday'),
             $this->rc->gettext('thursday'), $this->rc->gettext('friday'),
             $this->rc->gettext('saturday')
         );
         $settings['days_short'] = array(
             $this->rc->gettext('sun'), $this->rc->gettext('mon'),
             $this->rc->gettext('tue'), $this->rc->gettext('wed'),
             $this->rc->gettext('thu'), $this->rc->gettext('fri'),
             $this->rc->gettext('sat')
         );
         $settings['months'] = array(
             $this->rc->gettext('longjan'), $this->rc->gettext('longfeb'),
             $this->rc->gettext('longmar'), $this->rc->gettext('longapr'),
             $this->rc->gettext('longmay'), $this->rc->gettext('longjun'),
             $this->rc->gettext('longjul'), $this->rc->gettext('longaug'),
             $this->rc->gettext('longsep'), $this->rc->gettext('longoct'),
             $this->rc->gettext('longnov'), $this->rc->gettext('longdec')
         );
         $settings['months_short'] = array(
             $this->rc->gettext('jan'), $this->rc->gettext('feb'),
             $this->rc->gettext('mar'), $this->rc->gettext('apr'),
             $this->rc->gettext('may'), $this->rc->gettext('jun'),
             $this->rc->gettext('jul'), $this->rc->gettext('aug'),
             $this->rc->gettext('sep'), $this->rc->gettext('oct'),
             $this->rc->gettext('nov'), $this->rc->gettext('dec')
         );
         $settings['today'] = $this->rc->gettext('today');
 
         return $settings;
     }
 
 
     /**
      * Helper function to set date/time format according to config and user preferences
      */
     private function date_format_defaults()
     {
         static $defaults = array();
 
         // nothing to be done
         if (isset($defaults['date_format']))
           return;
 
         $defaults['date_format'] = $this->rc->config->get('calendar_date_format', $this->rc->config->get('date_format'));
         $defaults['time_format'] = $this->rc->config->get('calendar_time_format', $this->rc->config->get('time_format'));
 
         // override defaults
         if ($defaults['date_format'])
             $this->defaults['calendar_date_format'] = $defaults['date_format'];
         if ($defaults['time_format'])
             $this->defaults['calendar_time_format'] = $defaults['time_format'];
 
         // derive format variants from basic date format
         $format_sets = $this->rc->config->get('calendar_date_format_sets', $this->defaults['calendar_date_format_sets']);
         if ($format_set = $format_sets[$this->defaults['calendar_date_format']]) {
             $this->defaults['calendar_date_long'] = $format_set[0];
             $this->defaults['calendar_date_short'] = $format_set[1];
             $this->defaults['calendar_date_agenda'] = $format_set[2];
         }
     }
 
     /**
      * Compose a date string for the given event
      */
     public function event_date_text($event)
     {
         $fromto  = '--';
         $is_task = !empty($event['_type']) && $event['_type'] == 'task';
 
         $this->date_format_defaults();
 
         $date_format = self::to_php_date_format($this->rc->config->get('calendar_date_format', $this->defaults['calendar_date_format']));
         $time_format = self::to_php_date_format($this->rc->config->get('calendar_time_format', $this->defaults['calendar_time_format']));
 
         $getTimezone = function ($date) {
             if ($newTz = $date->getTimezone()) {
                 return $newTz->getName();
             }
 
             return '';
         };
 
         $formatDate = function ($date, $format) use ($getTimezone) {
             // This is a workaround for the rcmail::format_date() which does not play nice with timezone
             $tz = $this->rc->config->get('timezone');
             if ($dateTz = $getTimezone($date)) {
                 $this->rc->config->set('timezone', $dateTz);
             }
             $result = $this->rc->format_date($date, $format);
             $this->rc->config->set('timezone', $tz);
 
             return $result;
         };
 
         // handle task objects
         if ($is_task && !empty($event['due']) && is_object($event['due'])) {
             $fromto = $formatDate($event['due'], !empty($event['due']->_dateonly) ? $date_format : null);
 
             // add timezone information
             if ($fromto && empty($event['due']->_dateonly) && ($tz = $getTimezone($event['due']))) {
                 $fromto .= ' (' . strtr($tz, '_', ' ') . ')';
             }
 
             return $fromto;
         }
 
         // abort if no valid event dates are given
         if (!is_object($event['start']) || !is_a($event['start'], 'DateTime') || !is_object($event['end']) || !is_a($event['end'], 'DateTime')) {
             return $fromto;
         }
 
         if ($event['allday']) {
             $fromto = $formatDate($event['start'], $date_format);
             if (($todate = $formatDate($event['end'], $date_format)) != $fromto) {
                 $fromto .= ' - ' . $todate;
             }
         }
         else if ($event['start']->format('Ymd') === $event['end']->format('Ymd')) {
             $fromto = $formatDate($event['start'], $date_format) . ' ' . $formatDate($event['start'], $time_format) .
                 ' - ' . $formatDate($event['end'], $time_format);
         }
         else {
             $fromto = $formatDate($event['start'], $date_format) . ' ' . $formatDate($event['start'], $time_format) .
                 ' - ' . $formatDate($event['end'], $date_format) . ' ' . $formatDate($event['end'], $time_format);
         }
 
         // add timezone information
         if ($fromto && empty($event['allday']) && ($tz = $getTimezone($event['start']))) {
             $fromto .= ' (' . strtr($tz, '_', ' ') . ')';
         }
 
         return $fromto;
     }
 
 
     /**
      * Render HTML form for alarm configuration
      */
     public function alarm_select($attrib, $alarm_types, $absolute_time = true)
     {
         unset($attrib['name']);
 
         $input_value    = new html_inputfield(array('name' => 'alarmvalue[]', 'class' => 'edit-alarm-value form-control', 'size' => 3));
         $input_date     = new html_inputfield(array('name' => 'alarmdate[]', 'class' => 'edit-alarm-date form-control', 'size' => 10));
         $input_time     = new html_inputfield(array('name' => 'alarmtime[]', 'class' => 'edit-alarm-time form-control', 'size' => 6));
         $select_type    = new html_select(array('name' => 'alarmtype[]', 'class' => 'edit-alarm-type form-control', 'id' => $attrib['id']));
         $select_offset  = new html_select(array('name' => 'alarmoffset[]', 'class' => 'edit-alarm-offset form-control'));
         $select_related = new html_select(array('name' => 'alarmrelated[]', 'class' => 'edit-alarm-related form-control'));
         $object_type    = !empty($attrib['_type']) ? $attrib['_type'] : 'event';
 
         $select_type->add($this->gettext('none'), '');
         foreach ($alarm_types as $type) {
             $select_type->add($this->gettext(strtolower("alarm{$type}option")), $type);
         }
 
         foreach (array('-M','-H','-D','+M','+H','+D') as $trigger) {
             $select_offset->add($this->gettext('trigger' . $trigger), $trigger);
         }
 
         $select_offset->add($this->gettext('trigger0'), '0');
         if ($absolute_time) {
             $select_offset->add($this->gettext('trigger@'), '@');
         }
 
         $select_related->add($this->gettext('relatedstart'), 'start');
         $select_related->add($this->gettext('relatedend' . $object_type), 'end');
 
         // pre-set with default values from user settings
         $preset = self::parse_alarm_value($this->rc->config->get('calendar_default_alarm_offset', '-15M'));
         $hidden = array('style' => 'display:none');
 
         return html::span('edit-alarm-set',
             $select_type->show($this->rc->config->get('calendar_default_alarm_type', '')) . ' ' .
             html::span(array('class' => 'edit-alarm-values input-group', 'style' => 'display:none'),
                 $input_value->show($preset[0]) . ' ' .
                 $select_offset->show($preset[1]) . ' ' .
                 $select_related->show() . ' ' .
                 $input_date->show('', $hidden) . ' ' .
                 $input_time->show('', $hidden)
             )
         );
     }
 
     /**
      * Get a list of email addresses of the given user (from login and identities)
      *
      * @param string User Email (default to current user)
      *
      * @return array Email addresses related to the user
      */
     public function get_user_emails($user = null)
     {
         static $_emails = array();
 
         if (empty($user)) {
             $user = $this->rc->user->get_username();
         }
 
         // return cached result
         if (isset($_emails[$user])) {
             return $_emails[$user];
         }
 
         $emails = array($user);
         $plugin = $this->rc->plugins->exec_hook('calendar_user_emails', array('emails' => $emails));
         $emails = array_map('strtolower', $plugin['emails']);
 
         // add all emails from the current user's identities
         if (!$plugin['abort'] && ($user == $this->rc->user->get_username())) {
             foreach ($this->rc->user->list_emails() as $identity) {
                 $emails[] = strtolower($identity['email']);
             }
         }
 
         $_emails[$user] = array_unique($emails);
         return $_emails[$user];
     }
 
     /**
      * Set the given participant status to the attendee matching the current user's identities
      * Unsets 'rsvp' flag too.
      *
      * @param array  &$event    Event data
      * @param string $status    The PARTSTAT value to set
      * @param bool   $recursive Recurive call
      *
      * @return mixed Email address of the updated attendee or False if none matching found
      */
     public function set_partstat(&$event, $status, $recursive = true)
     {
         $success = false;
         $emails = $this->get_user_emails();
         foreach ((array)$event['attendees'] as $i => $attendee) {
             if ($attendee['email'] && in_array(strtolower($attendee['email']), $emails)) {
                 $event['attendees'][$i]['status'] = strtoupper($status);
                 unset($event['attendees'][$i]['rsvp']);
                 $success = $attendee['email'];
             }
         }
 
         // apply partstat update to each existing exception
         if ($event['recurrence'] && is_array($event['recurrence']['EXCEPTIONS'])) {
             foreach ($event['recurrence']['EXCEPTIONS'] as $i => $exception) {
                 $this->set_partstat($event['recurrence']['EXCEPTIONS'][$i], $status, false);
             }
 
             // set link to top-level exceptions
             $event['exceptions'] = &$event['recurrence']['EXCEPTIONS'];
         }
 
         return $success;
     }
 
 
     /*********  Alarms handling  *********/
 
     /**
      * Helper function to convert alarm trigger strings
      * into two-field values (e.g. "-45M" => 45, "-M")
      */
     public static function parse_alarm_value($val)
     {
         if ($val[0] == '@') {
             return array(new DateTime($val));
         }
         else if (preg_match('/([+-]?)P?(T?\d+[HMSDW])+/', $val, $m) && preg_match_all('/T?(\d+)([HMSDW])/', $val, $m2, PREG_SET_ORDER)) {
             if ($m[1] == '')
                 $m[1] = '+';
             foreach ($m2 as $seg) {
                 $prefix = $seg[2] == 'D' || $seg[2] == 'W' ? 'P' : 'PT';
                 if ($seg[1] > 0) {  // ignore zero values
                     // convert seconds to minutes
                     if ($seg[2] == 'S') {
                         $seg[2] = 'M';
                         $seg[1] = max(1, round($seg[1]/60));
                     }
 
                     return array($seg[1], $m[1].$seg[2], $m[1].$seg[1].$seg[2], $m[1].$prefix.$seg[1].$seg[2]);
                 }
             }
 
             // return zero value nevertheless
             return array($seg[1], $m[1].$seg[2], $m[1].$seg[1].$seg[2], $m[1].$prefix.$seg[1].$seg[2]);
         }
 
         return false;
     }
 
     /**
      * Convert the alarms list items to be processed on the client
      */
     public static function to_client_alarms($valarms)
     {
         return array_map(function($alarm) {
             if ($alarm['trigger'] instanceof DateTimeInterface) {
                 $alarm['trigger'] = '@' . $alarm['trigger']->format('U');
             }
             else if ($trigger = libcalendaring::parse_alarm_value($alarm['trigger'])) {
                 $alarm['trigger'] = $trigger[2];
             }
             return $alarm;
         }, (array)$valarms);
     }
 
     /**
      * Process the alarms values submitted by the client
      */
     public static function from_client_alarms($valarms)
     {
         return array_map(function($alarm){
             if ($alarm['trigger'][0] == '@') {
                 try {
                     $alarm['trigger'] = new DateTime($alarm['trigger']);
                     $alarm['trigger']->setTimezone(new DateTimeZone('UTC'));
                 }
                 catch (Exception $e) { /* handle this ? */ }
             }
             else if ($trigger = libcalendaring::parse_alarm_value($alarm['trigger'])) {
                 $alarm['trigger'] = $trigger[3];
             }
             return $alarm;
         }, (array)$valarms);
     }
 
     /**
      * Render localized text for alarm settings
      */
     public static function alarms_text($alarms)
     {
         if (is_array($alarms) && is_array($alarms[0])) {
             $texts = array();
             foreach ($alarms as $alarm) {
                 if ($text = self::alarm_text($alarm))
                     $texts[] = $text;
             }
 
             return join(', ', $texts);
         }
         else {
             return self::alarm_text($alarms);
         }
     }
 
     /**
      * Render localized text for a single alarm property
      */
     public static function alarm_text($alarm)
     {
         $related = null;
 
         if (is_string($alarm)) {
             list($trigger, $action) = explode(':', $alarm);
         }
         else {
             $trigger = $alarm['trigger'];
             $action  = $alarm['action'];
 
             if (!empty($alarm['related'])) {
                 $related = $alarm['related'];
             }
         }
 
         $text  = '';
         $rcube = rcube::get_instance();
 
         switch ($action) {
         case 'EMAIL':
             $text = $rcube->gettext('libcalendaring.alarmemail');
             break;
         case 'DISPLAY':
             $text = $rcube->gettext('libcalendaring.alarmdisplay');
             break;
         case 'AUDIO':
             $text = $rcube->gettext('libcalendaring.alarmaudio');
             break;
         }
 
         if ($trigger instanceof DateTimeInterface) {
             $text .= ' ' . $rcube->gettext(array(
                 'name' => 'libcalendaring.alarmat',
                 'vars' => array('datetime' => $rcube->format_date($trigger))
             ));
         }
         else if (preg_match('/@(\d+)/', $trigger, $m)) {
             $text .= ' ' . $rcube->gettext(array(
                 'name' => 'libcalendaring.alarmat',
                 'vars' => array('datetime' => $rcube->format_date($m[1]))
             ));
         }
         else if ($val = self::parse_alarm_value($trigger)) {
             $r = $related && strtoupper($related) == 'END' ? 'end' : '';
             // TODO: for all-day events say 'on date of event at XX' ?
             if ($val[0] == 0) {
                 $text .= ' ' . $rcube->gettext('libcalendaring.triggerattime' . $r);
             }
             else {
                 $label = 'libcalendaring.trigger' . $r . $val[1];
                 $text .= ' ' . intval($val[0]) . ' ' . $rcube->gettext($label);
             }
         }
         else {
             return false;
         }
 
         return $text;
     }
 
     /**
      * Get the next alarm (time & action) for the given event
      *
      * @param array Record data
      * @return array Hash array with alarm time/type or null if no alarms are configured
      */
     public static function get_next_alarm($rec, $type = 'event')
     {
         if (
             (empty($rec['valarms']) && empty($rec['alarms']))
             || !empty($rec['cancelled'])
             || (!empty($rec['status']) && $rec['status'] == 'CANCELLED')
         ) {
             return null;
         }
 
         if ($type == 'task') {
             $timezone = self::get_instance()->timezone;
             if (!empty($rec['startdate'])) {
                 $time = !empty($rec['starttime']) ? $rec['starttime'] : '12:00';
                 $rec['start'] = new DateTime($rec['startdate'] . ' ' . $time, $timezone);
             }
             if (!empty($rec['date'])) {
                 $time = !empty($rec['time']) ? $rec['time'] : '12:00';
                 $rec[!empty($rec['start']) ? 'end' : 'start'] = new DateTime($rec['date'] . ' ' . $time, $timezone);
             }
         }
 
         if (empty($rec['end'])) {
             $rec['end'] = $rec['start'];
         }
 
         // support legacy format
         if (empty($rec['valarms'])) {
             list($trigger, $action) = explode(':', $rec['alarms'], 2);
             if ($alarm = self::parse_alarm_value($trigger)) {
                 $rec['valarms'] = array(array('action' => $action, 'trigger' => $alarm[3] ?: $alarm[0]));
             }
         }
 
         // alarm ID eq. record ID by default to keep backwards compatibility
         $alarm_id   = isset($rec['id']) ? $rec['id'] : null;
         $alarm_prop = null;
         $expires    = new DateTime('now - 12 hours');
         $notify_at  = null;
 
         // handle multiple alarms
         foreach ($rec['valarms'] as $alarm) {
             $notify_time = null;
 
             if ($alarm['trigger'] instanceof DateTimeInterface) {
                 $notify_time = $alarm['trigger'];
             }
             else if (is_string($alarm['trigger'])) {
                 $refdate = !empty($alarm['related']) && $alarm['related'] == 'END' ? $rec['end'] : $rec['start'];
 
                 // abort if no reference date is available to compute notification time
                 if (!is_a($refdate, 'DateTime')) {
                     continue;
                 }
 
                 // TODO: for all-day events, take start @ 00:00 as reference date ?
 
                 try {
                     $interval = new DateInterval(trim($alarm['trigger'], '+-'));
                     $interval->invert = $alarm['trigger'][0] == '-';
                     $notify_time = clone $refdate;
                     $notify_time->add($interval);
                 }
                 catch (Exception $e) {
                     rcube::raise_error($e, true);
                     continue;
                 }
             }
 
             if ($notify_time && (!$notify_at || ($notify_time > $notify_at && $notify_time > $expires))) {
                 $notify_at  = $notify_time;
                 $action     = isset($alarm['action']) ? $alarm['action'] : null;
                 $alarm_prop = $alarm;
 
                 // generate a unique alarm ID if multiple alarms are set
                 if (count($rec['valarms']) > 1) {
                     $rec_id = substr(md5(isset($rec['id']) ? $rec['id'] : 'none'), 0, 16);
                     $alarm_id = $rec_id . '-' . $notify_at->format('Ymd\THis');
                 }
             }
         }
 
         return !$notify_at ? null : array(
             'time'   => $notify_at->format('U'),
             'action' => !empty($action) ? strtoupper($action) : 'DISPLAY',
             'id'     => $alarm_id,
             'prop'   => $alarm_prop,
         );
     }
 
     /**
      * Handler for keep-alive requests
      * This will check for pending notifications and pass them to the client
      */
     public function refresh($attr)
     {
         // collect pending alarms from all providers (e.g. calendar, tasks)
         $plugin = $this->rc->plugins->exec_hook('pending_alarms', array(
             'time' => time(),
             'alarms' => array(),
         ));
 
         if (!$plugin['abort'] && !empty($plugin['alarms'])) {
             // make sure texts and env vars are available on client
             $this->add_texts('localization/', true);
             $this->rc->output->add_label('close');
             $this->rc->output->set_env('snooze_select', $this->snooze_select());
             $this->rc->output->command('plugin.display_alarms', $this->_alarms_output($plugin['alarms']));
         }
     }
 
     /**
      * Handler for alarm dismiss/snooze requests
      */
     public function alarms_action()
     {
 //        $action = rcube_utils::get_input_value('action', rcube_utils::INPUT_GPC);
         $data  = rcube_utils::get_input_value('data', rcube_utils::INPUT_POST, true);
 
         $data['ids'] = explode(',', $data['id']);
         $plugin = $this->rc->plugins->exec_hook('dismiss_alarms', $data);
 
         if (!empty($plugin['success'])) {
             $this->rc->output->show_message('successfullysaved', 'confirmation');
         }
         else {
             $this->rc->output->show_message('calendar.errorsaving', 'error');
         }
     }
 
     /**
      * Generate reduced and streamlined output for pending alarms
      */
     private function _alarms_output($alarms)
     {
         $out = array();
         foreach ($alarms as $alarm) {
             $out[] = array(
                 'id'       => $alarm['id'],
                 'start'    => !empty($alarm['start']) ? $this->adjust_timezone($alarm['start'])->format('c') : '',
                 'end'      => !empty($alarm['end'])? $this->adjust_timezone($alarm['end'])->format('c') : '',
                 'allDay'   => !empty($alarm['allday']),
                 'action'   => $alarm['action'],
                 'title'    => $alarm['title'],
                 'location' => $alarm['location'],
                 'calendar' => $alarm['calendar'],
             );
         }
 
         return $out;
     }
 
     /**
      * Render a dropdown menu to choose snooze time
      */
     private function snooze_select($attrib = array())
     {
         $steps = array(
              5 => 'repeatinmin',
             10 => 'repeatinmin',
             15 => 'repeatinmin',
             20 => 'repeatinmin',
             30 => 'repeatinmin',
             60 => 'repeatinhr',
             120 => 'repeatinhrs',
             1440 => 'repeattomorrow',
             10080 => 'repeatinweek',
         );
 
         $items = array();
         foreach ($steps as $n => $label) {
             $items[] = html::tag('li', null, html::a(array('href' => "#" . ($n * 60), 'class' => 'active'),
                 $this->gettext(array('name' => $label, 'vars' => array('min' => $n % 60, 'hrs' => intval($n / 60))))));
         }
 
         return html::tag('ul', $attrib + array('class' => 'toolbarmenu menu'), join("\n", $items), html::$common_attrib);
     }
 
 
     /*********  Recurrence rules handling ********/
 
     /**
      * Render localized text describing the recurrence rule of an event
      */
     public function recurrence_text($rrule)
     {
         $limit     = 10;
         $exdates   = array();
         $format    = $this->rc->config->get('calendar_date_format', $this->defaults['calendar_date_format']);
         $format    = self::to_php_date_format($format);
         $format_fn = function($dt) use ($format) {
             return rcmail::get_instance()->format_date($dt, $format);
         };
 
         if (!empty($rrule['EXDATE']) && is_array($rrule['EXDATE'])) {
             $exdates = array_map($format_fn, $rrule['EXDATE']);
         }
 
         if (empty($rrule['FREQ']) && !empty($rrule['RDATE'])) {
             $rdates = array_map($format_fn, $rrule['RDATE']);
             $more   = false;
 
             if (!empty($exdates)) {
                 $rdates = array_diff($rdates, $exdates);
             }
 
             if (count($rdates) > $limit) {
                 $rdates = array_slice($rdates, 0, $limit);
                 $more   = true;
             }
 
             return $this->gettext('ondate') . ' ' . join(', ', $rdates) . ($more ? '...' : '');
         }
 
         $output  = sprintf('%s %d ', $this->gettext('every'), $rrule['INTERVAL'] ?: 1);
 
         switch ($rrule['FREQ']) {
         case 'DAILY':
             $output .= $this->gettext('days');
             break;
         case 'WEEKLY':
             $output .= $this->gettext('weeks');
             break;
         case 'MONTHLY':
             $output .= $this->gettext('months');
             break;
         case 'YEARLY':
             $output .= $this->gettext('years');
             break;
         }
 
         if (!empty($rrule['COUNT'])) {
             $until = $this->gettext(array('name' => 'forntimes', 'vars' => array('nr' => $rrule['COUNT'])));
         }
         else if (!empty($rrule['UNTIL'])) {
             $until = $this->gettext('recurrencend') . ' ' . $this->rc->format_date($rrule['UNTIL'], $format);
         }
         else {
             $until = $this->gettext('forever');
         }
 
         $output .= ', ' . $until;
 
         if (!empty($exdates)) {
             $more = false;
             if (count($exdates) > $limit) {
                 $exdates = array_slice($exdates, 0, $limit);
                 $more    = true;
             }
 
             $output  .= '; ' . $this->gettext('except') . ' ' . join(', ', $exdates) . ($more ? '...' : '');
         }
 
         return $output;
     }
 
     /**
      * Generate the form for recurrence settings
      */
     public function recurrence_form($attrib = array())
     {
         switch ($attrib['part']) {
             // frequency selector
             case 'frequency':
                 $select = new html_select(array('name' => 'frequency', 'id' => 'edit-recurrence-frequency', 'class' => 'form-control'));
                 $select->add($this->gettext('never'),   '');
                 $select->add($this->gettext('daily'),   'DAILY');
                 $select->add($this->gettext('weekly'),  'WEEKLY');
                 $select->add($this->gettext('monthly'), 'MONTHLY');
                 $select->add($this->gettext('yearly'),  'YEARLY');
                 $select->add($this->gettext('rdate'),   'RDATE');
                 $html = html::label(array('for' => 'edit-recurrence-frequency', 'class' => 'col-form-label col-sm-2'), $this->gettext('frequency'))
                     . html::div('col-sm-10', $select->show(''));
                 break;
 
             // daily recurrence
             case 'daily':
                 $select = $this->interval_selector(array('name' => 'interval', 'class' => 'edit-recurrence-interval form-control', 'id' => 'edit-recurrence-interval-daily'));
                 $html = html::div($attrib, html::label(array('for' => 'edit-recurrence-interval-daily', 'class' => 'col-form-label col-sm-2'), $this->gettext('every'))
                     . html::div('col-sm-10 input-group', $select->show(1) . html::span('label-after input-group-append', html::span('input-group-text', $this->gettext('days')))));
                 break;
 
             // weekly recurrence form
             case 'weekly':
                 $select = $this->interval_selector(array('name' => 'interval', 'class' => 'edit-recurrence-interval form-control', 'id' => 'edit-recurrence-interval-weekly'));
                 $html = html::div($attrib, html::label(array('for' => 'edit-recurrence-interval-weekly', 'class' => 'col-form-label col-sm-2'), $this->gettext('every'))
                     . html::div('col-sm-10 input-group', $select->show(1) . html::span('label-after input-group-append', html::span('input-group-text', $this->gettext('weeks')))));
 
                 // weekday selection
                 $daymap   = array('sun','mon','tue','wed','thu','fri','sat');
                 $checkbox = new html_checkbox(array('name' => 'byday', 'class' => 'edit-recurrence-weekly-byday'));
                 $first    = $this->rc->config->get('calendar_first_day', 1);
 
                 for ($weekdays = '', $j = $first; $j <= $first+6; $j++) {
                     $d = $j % 7;
                     $weekdays .= html::label(array('class' => 'weekday'),
                         $checkbox->show('', array('value' => strtoupper(substr($daymap[$d], 0, 2)))) .
                         $this->gettext($daymap[$d])
                     ) . ' ';
                 }
 
                 $html .= html::div($attrib, html::label(array('class' => 'col-form-label col-sm-2'), $this->gettext('bydays'))
                     . html::div('col-sm-10 form-control-plaintext', $weekdays));
                 break;
 
             // monthly recurrence form
             case 'monthly':
                 $select = $this->interval_selector(array('name' => 'interval', 'class' => 'edit-recurrence-interval form-control', 'id' => 'edit-recurrence-interval-monthly'));
                 $html = html::div($attrib, html::label(array('for' => 'edit-recurrence-interval-monthly', 'class' => 'col-form-label col-sm-2'), $this->gettext('every'))
                     . html::div('col-sm-10 input-group', $select->show(1) . html::span('label-after input-group-append', html::span('input-group-text', $this->gettext('months')))));
 
                 $checkbox = new html_checkbox(array('name' => 'bymonthday', 'class' => 'edit-recurrence-monthly-bymonthday'));
                 for ($monthdays = '', $d = 1; $d <= 31; $d++) {
                     $monthdays .= html::label(array('class' => 'monthday'), $checkbox->show('', array('value' => $d)) . $d);
                     $monthdays .= $d % 7 ? ' ' : html::br();
                 }
 
                 // rule selectors
                 $radio = new html_radiobutton(array('name' => 'repeatmode', 'class' => 'edit-recurrence-monthly-mode'));
                 $table = new html_table(array('cols' => 2, 'border' => 0, 'cellpadding' => 0, 'class' => 'formtable'));
                 $table->add('label', html::label(null, $radio->show('BYMONTHDAY', array('value' => 'BYMONTHDAY')) . ' ' . $this->gettext('each')));
                 $table->add(null, $monthdays);
                 $table->add('label', html::label(null, $radio->show('', array('value' => 'BYDAY')) . ' ' . $this->gettext('every')));
                 $table->add('recurrence-onevery', $this->rrule_selectors($attrib['part']));
 
                 $html .= html::div($attrib, html::label(array('class' => 'col-form-label col-sm-2'), $this->gettext('bydays'))
                     . html::div('col-sm-10 form-control-plaintext', $table->show()));
                 break;
 
             // annually recurrence form
             case 'yearly':
                 $select = $this->interval_selector(array('name' => 'interval', 'class' => 'edit-recurrence-interval form-control', 'id' => 'edit-recurrence-interval-yearly'));
                 $html = html::div($attrib, html::label(array('for' => 'edit-recurrence-interval-yearly', 'class' => 'col-form-label col-sm-2'), $this->gettext('every'))
                     . html::div('col-sm-10 input-group', $select->show(1) . html::span('label-after input-group-append', html::span('input-group-text', $this->gettext('years')))));
 
                 // month selector
                 $monthmap = array('','jan','feb','mar','apr','may','jun','jul','aug','sep','oct','nov','dec');
                 $checkbox = new html_checkbox(array('name' => 'bymonth', 'class' => 'edit-recurrence-yearly-bymonth'));
 
                 for ($months = '', $m = 1; $m <= 12; $m++) {
                     $months .= html::label(array('class' => 'month'), $checkbox->show(null, array('value' => $m)) . $this->gettext($monthmap[$m]));
                     $months .= $m % 4 ? ' ' : html::br();
                 }
 
                 $html .= html::div($attrib, html::label(array('class' => 'col-form-label col-sm-2'), $this->gettext('bymonths'))
                     . html::div('col-sm-10 form-control-plaintext',
                         html::div(array('id' => 'edit-recurrence-yearly-bymonthblock'), $months)
                         . html::div('recurrence-onevery', $this->rrule_selectors($attrib['part'], '---'))
                     ));
                 break;
 
             // end of recurrence form
             case 'until':
                 $radio  = new html_radiobutton(array('name' => 'repeat', 'class' => 'edit-recurrence-until'));
                 $select = $this->interval_selector(array('name' => 'times', 'id' => 'edit-recurrence-repeat-times', 'class' => 'form-control'));
                 $input  = new html_inputfield(array('name' => 'untildate', 'id' => 'edit-recurrence-enddate', 'size' => '10', 'class' => 'form-control datepicker'));
 
                 $html = html::div('line first',
                     $radio->show('', array('value' => '', 'id' => 'edit-recurrence-repeat-forever'))
                         . ' ' . html::label('edit-recurrence-repeat-forever', $this->gettext('forever'))
                 );
 
                 $label = $this->gettext('ntimes');
                 if (strpos($label, '$') === 0) {
                     $label = str_replace('$n', '', $label);
                     $group  = $select->show(1)
                         . html::span('input-group-append', html::span('input-group-text', rcube::Q($label)));
                 }
                 else {
                     $label = str_replace('$n', '', $label);
                     $group  = html::span('input-group-prepend', html::span('input-group-text', rcube::Q($label)))
                         . $select->show(1);
                 }
 
                 $html .= html::div('line',
                     $radio->show('', array('value' => 'count', 'id' => 'edit-recurrence-repeat-count'))
                         . ' ' . html::label('edit-recurrence-repeat-count', $this->gettext('for'))
                         . ' ' . html::span('input-group', $group)
                 );
 
                 $html .= html::div('line',
                     $radio->show('', array('value' => 'until', 'id' => 'edit-recurrence-repeat-until', 'aria-label' => $this->gettext('untilenddate')))
                         . ' ' . html::label('edit-recurrence-repeat-until', $this->gettext('untildate'))
                         . ' ' . $input->show('', array('aria-label' => $this->gettext('untilenddate')))
                 );
 
                 $html = html::div($attrib, html::label(array('class' => 'col-form-label col-sm-2'), ucfirst($this->gettext('recurrencend')))
                     . html::div('col-sm-10', $html));
                 break;
 
             case 'rdate':
                 $ul     = html::tag('ul', array('id' => 'edit-recurrence-rdates', 'class' => 'recurrence-rdates'), '');
                 $input  = new html_inputfield(array('name' => 'rdate', 'id' => 'edit-recurrence-rdate-input', 'size' => "10", 'class' => 'form-control datepicker'));
                 $button = new html_inputfield(array('type' => 'button', 'class' => 'button add', 'value' => $this->gettext('addrdate')));
 
                 $html = html::div($attrib, html::label(array('class' => 'col-form-label col-sm-2', 'for' => 'edit-recurrence-rdate-input'), $this->gettext('bydates'))
                     . html::div('col-sm-10', $ul . html::div('inputform', $input->show() . $button->show())));
                 break;
         }
 
         return $html;
     }
 
     /**
      * Input field for interval selection
      */
     private function interval_selector($attrib)
     {
         $select = new html_select($attrib);
         $select->add(range(1,30), range(1,30));
         return $select;
     }
 
     /**
      * Drop-down menus for recurrence rules like "each last sunday of"
      */
     private function rrule_selectors($part, $noselect = null)
     {
         // rule selectors
         $select_prefix = new html_select(array('name' => 'bydayprefix', 'id' => "edit-recurrence-$part-prefix", 'class' => 'form-control'));
         if ($noselect) $select_prefix->add($noselect, '');
         $select_prefix->add(array(
                 $this->gettext('first'),
                 $this->gettext('second'),
                 $this->gettext('third'),
                 $this->gettext('fourth'),
                 $this->gettext('last')
             ),
             array(1, 2, 3, 4, -1));
 
         $select_wday = new html_select(array('name' => 'byday', 'id' => "edit-recurrence-$part-byday", 'class' => 'form-control'));
         if ($noselect) $select_wday->add($noselect, '');
 
         $daymap = array('sunday','monday','tuesday','wednesday','thursday','friday','saturday');
         $first = $this->rc->config->get('calendar_first_day', 1);
         for ($j = $first; $j <= $first+6; $j++) {
             $d = $j % 7;
             $select_wday->add($this->gettext($daymap[$d]), strtoupper(substr($daymap[$d], 0, 2)));
         }
 
         return $select_prefix->show() . '&nbsp;' . $select_wday->show();
     }
 
     /**
      * Convert the recurrence settings to be processed on the client
      */
     public function to_client_recurrence($recurrence, $allday = false)
     {
         if (!empty($recurrence['UNTIL'])) {
             $recurrence['UNTIL'] = $this->adjust_timezone($recurrence['UNTIL'], $allday)->format('c');
         }
 
         // format RDATE values
         if (!empty($recurrence['RDATE'])) {
             $libcal = $this;
             $recurrence['RDATE'] = array_map(function($rdate) use ($libcal) {
                 return $libcal->adjust_timezone($rdate, true)->format('c');
             }, (array) $recurrence['RDATE']);
         }
 
         unset($recurrence['EXCEPTIONS']);
 
         return $recurrence;
     }
 
     /**
      * Process the alarms values submitted by the client
      */
     public function from_client_recurrence($recurrence, $start = null)
     {
         if (is_array($recurrence) && !empty($recurrence['UNTIL'])) {
             $recurrence['UNTIL'] = new DateTime($recurrence['UNTIL'], $this->timezone);
         }
 
         if (is_array($recurrence) && !empty($recurrence['RDATE'])) {
             $tz = $this->timezone;
             $recurrence['RDATE'] = array_map(function($rdate) use ($tz, $start) {
                 try {
                     $dt = new DateTime($rdate, $tz);
                     if (is_a($start, 'DateTime'))
                         $dt->setTime($start->format('G'), $start->format('i'));
                     return $dt;
                 }
                 catch (Exception $e) {
                     return null;
                 }
             }, $recurrence['RDATE']);
         }
 
         return $recurrence;
     }
 
 
     /*********  iTip message detection  *********/
 
     /**
      * Check mail message structure of there are .ics files attached
      */
     public function mail_message_load($p)
     {
         $this->ical_message = $p['object'];
         $itip_part          = null;
 
         // check all message parts for .ics files
         foreach ((array)$this->ical_message->mime_parts as $part) {
             if (self::part_is_vcalendar($part, $this->ical_message)) {
                 if (!empty($part->ctype_parameters['method'])) {
                     $itip_part = $part->mime_id;
                 }
                 else {
                     $this->ical_parts[] = $part->mime_id;
                 }
             }
         }
 
         // priorize part with method parameter
         if ($itip_part) {
             $this->ical_parts = array($itip_part);
         }
     }
 
     /**
      * Getter for the parsed iCal objects attached to the current email message
      *
      * @return object libcalendaring_vcalendar parser instance with the parsed objects
      */
     public function get_mail_ical_objects()
     {
         // create parser and load ical objects
         if (!$this->mail_ical_parser) {
             $this->mail_ical_parser = $this->get_ical();
 
             foreach ($this->ical_parts as $mime_id) {
                 $part    = $this->ical_message->mime_parts[$mime_id];
                 $charset = $part->ctype_parameters['charset'] ?: RCUBE_CHARSET;
                 $this->mail_ical_parser->import($this->ical_message->get_part_body($mime_id, true), $charset);
 
                 // check if the parsed object is an instance of a recurring event/task
                 array_walk($this->mail_ical_parser->objects, 'libcalendaring::identify_recurrence_instance');
 
                 // stop on the part that has an iTip method specified
                 if (count($this->mail_ical_parser->objects) && $this->mail_ical_parser->method) {
                     $this->mail_ical_parser->message_date = $this->ical_message->headers->date;
                     $this->mail_ical_parser->mime_id = $mime_id;
 
                     // store the message's sender address for comparisons
                     $from = rcube_mime::decode_address_list($this->ical_message->headers->from, 1, true, null, true);
                     $this->mail_ical_parser->sender = !empty($from) ? $from[1] : '';
 
                     if (!empty($this->mail_ical_parser->sender)) {
                         foreach ($this->mail_ical_parser->objects as $i => $object) {
                             $this->mail_ical_parser->objects[$i]['_sender'] = $this->mail_ical_parser->sender;
                             $this->mail_ical_parser->objects[$i]['_sender_utf'] = rcube_utils::idn_to_utf8($this->mail_ical_parser->sender);
                         }
                     }
 
                     break;
                 }
             }
         }
 
         return $this->mail_ical_parser;
     }
 
     /**
      * Read the given mime message from IMAP and parse ical data
      *
      * @param string Mailbox name
      * @param string Message UID
      * @param string Message part ID and object index (e.g. '1.2:0')
      * @param string Object type filter (optional)
      *
      * @return array Hash array with the parsed iCal 
      */
     public function mail_get_itip_object($mbox, $uid, $mime_id, $type = null)
     {
         $charset = RCUBE_CHARSET;
 
         // establish imap connection
         $imap = $this->rc->get_storage();
         $imap->set_folder($mbox);
 
         if ($uid && $mime_id) {
             list($mime_id, $index) = explode(':', $mime_id);
 
             $part    = $imap->get_message_part($uid, $mime_id);
             $headers = $imap->get_message_headers($uid);
             $parser  = $this->get_ical();
 
             if (!empty($part->ctype_parameters['charset'])) {
                 $charset = $part->ctype_parameters['charset'];
             }
 
             if ($part) {
                 $objects = $parser->import($part, $charset);
             }
         }
 
         // successfully parsed events/tasks?
         if (!empty($objects) && ($object = $objects[$index]) && (!$type || $object['_type'] == $type)) {
             if ($parser->method)
                 $object['_method'] = $parser->method;
 
             // store the message's sender address for comparisons
             $from = rcube_mime::decode_address_list($headers->from, 1, true, null, true);
             $object['_sender'] = !empty($from) ? $from[1] : '';
             $object['_sender_utf'] = rcube_utils::idn_to_utf8($object['_sender']);
 
             // check if this is an instance of a recurring event/task
             self::identify_recurrence_instance($object);
 
             return $object;
         }
 
         return null;
     }
 
     /**
      * Checks if specified message part is a vcalendar data
      *
      * @param rcube_message_part Part object
      * @param rcube_message      Message object
      *
      * @return boolean True if part is of type vcard
      */
     public static function part_is_vcalendar($part, $message = null)
     {
         // First check if the message is "valid" (i.e. not multipart/report)
         if ($message) {
             $level = explode('.', $part->mime_id);
 
             while (array_pop($level) !== null) {
                 $id     = join('.', $level) ?: 0;
                 $parent = !empty($message->mime_parts[$id]) ? $message->mime_parts[$id] : null;
                 if ($parent && $parent->mimetype == 'multipart/report') {
                     return false;
                 }
             }
         }
 
         return (
             in_array($part->mimetype, array('text/calendar', 'text/x-vcalendar', 'application/ics')) ||
             // Apple sends files as application/x-any (!?)
             ($part->mimetype == 'application/x-any' && !empty($part->filename) && preg_match('/\.ics$/i', $part->filename))
         );
     }
 
     /**
      * Single occourrences of recurring events are identified by their RECURRENCE-ID property
      * in iCal which is represented as 'recurrence_date' in our internal data structure.
      *
      * Check if such a property exists and derive the '_instance' identifier and '_savemode'
      * attributes which are used in the storage backend to identify the nested exception item.
      */
     public static function identify_recurrence_instance(&$object)
     {
         // for savemode=all, remove recurrence instance identifiers
         if (!empty($object['_savemode']) && $object['_savemode'] == 'all' && !empty($object['recurrence'])) {
             unset($object['_instance'], $object['recurrence_date']);
         }
         // set instance and 'savemode' according to recurrence-id
         else if (!empty($object['recurrence_date']) && $object['recurrence_date'] instanceof DateTimeInterface) {
             $object['_instance'] = self::recurrence_instance_identifier($object);
             $object['_savemode'] = !empty($object['thisandfuture']) ? 'future' : 'current';
         }
         else if (!empty($object['recurrence_id']) && !empty($object['_instance'])) {
             if (strlen($object['_instance']) > 4) {
                 $object['recurrence_date'] = rcube_utils::anytodatetime($object['_instance'], $object['start']->getTimezone());
             }
             else {
                 $object['recurrence_date'] = clone $object['start'];
             }
         }
     }
 
     /**
      * Return a date() format string to render identifiers for recurrence instances
      *
      * @param array Hash array with event properties
      * @return string Format string
      */
     public static function recurrence_id_format($event)
     {
         return !empty($event['allday']) ? 'Ymd' : 'Ymd\THis';
     }
 
     /**
      * Return the identifer for the given instance of a recurring event
      *
      * @param array Hash array with event properties
      * @param bool  All-day flag from the main event
      *
      * @return mixed Format string or null if identifier cannot be generated
      */
     public static function recurrence_instance_identifier($event, $allday = null)
     {
         $instance_date = !empty($event['recurrence_date']) ? $event['recurrence_date'] : $event['start'];
 
         if ($instance_date instanceof DateTimeInterface) {
             // According to RFC5545 (3.8.4.4) RECURRENCE-ID format should
             // be date/date-time depending on the main event type, not the exception
             if ($allday === null) {
                 $allday = !empty($event['allday']);
             }
 
             return $instance_date->format($allday ? 'Ymd' : 'Ymd\THis');
         }
     }
 
+    /**
+     * Check if a specified event is "identical" to the specified recurrence exception
+     *
+     * @param array Hash array with occurrence properties
+     * @param array Hash array with exception properties
+     *
+     * @return bool
+     */
+    public static function is_recurrence_exception($event, $exception)
+    {
+        $instance_date = !empty($event['recurrence_date']) ? $event['recurrence_date'] : $event['start'];
+        $exception_date = !empty($exception['recurrence_date']) ? $exception['recurrence_date'] : $exception['start'];
+
+        if ($instance_date instanceof DateTimeInterface && $exception_date instanceof DateTimeInterface) {
+            // Timezone???
+            return $instance_date->format('Ymd') === $exception_date->format('Ymd');
+        }
+
+        return false;
+    }
+
 
     /*********  Attendee handling functions  *********/
 
     /**
      * Handler for attendee group expansion requests
      */
     public function expand_attendee_group()
     {
         $id     = rcube_utils::get_input_value('id', rcube_utils::INPUT_POST);
         $data   = rcube_utils::get_input_value('data', rcube_utils::INPUT_POST, true);
         $result = array('id' => $id, 'members' => array());
         $maxnum = 500;
 
         // iterate over all autocomplete address books (we don't know the source of the group)
         foreach ((array)$this->rc->config->get('autocomplete_addressbooks', 'sql') as $abook_id) {
             if (($abook = $this->rc->get_address_book($abook_id)) && $abook->groups) {
                 foreach ($abook->list_groups($data['name'], 1) as $group) {
                     // this is the matching group to expand
                     if (in_array($data['email'], (array)$group['email'])) {
                         $abook->set_pagesize($maxnum);
                         $abook->set_group($group['ID']);
 
                         // get all members
                         $res = $abook->list_records($this->rc->config->get('contactlist_fields'));
 
                         // handle errors (e.g. sizelimit, timelimit)
                         if ($abook->get_error()) {
                             $result['error'] = $this->rc->gettext('expandattendeegrouperror', 'libcalendaring');
                             $res = false;
                         }
                         // check for maximum number of members (we don't wanna bloat the UI too much)
                         else if ($res->count > $maxnum) {
                             $result['error'] = $this->rc->gettext('expandattendeegroupsizelimit', 'libcalendaring');
                             $res = false;
                         }
 
                         while ($res && ($member = $res->iterate())) {
                             $emails = (array)$abook->get_col_values('email', $member, true);
                             if (!empty($emails) && ($email = array_shift($emails))) {
                                 $result['members'][] = array(
                                     'email' => $email,
                                     'name' => rcube_addressbook::compose_list_name($member),
                                 );
                             }
                         }
 
                         break 2;
                     }
                 }
             }
         }
 
         $this->rc->output->command('plugin.expand_attendee_callback', $result);
     }
 
     /**
      * Merge attendees of the old and new event version
      * with keeping current user and his delegatees status
      *
      * @param array &$new   New object data
      * @param array $old    Old object data
      * @param bool  $status New status of the current user
      */
     public function merge_attendees(&$new, $old, $status = null)
     {
         if (empty($status)) {
             $emails    = $this->get_user_emails();
             $delegates = array();
             $attendees = array();
 
             // keep attendee status of the current user
             foreach ((array) $new['attendees'] as $i => $attendee) {
                 if (empty($attendee['email'])) {
                     continue;
                 }
 
                 $attendees[] = $email = strtolower($attendee['email']);
 
                 if (in_array($email, $emails)) {
                     foreach ($old['attendees'] as $_attendee) {
                         if ($attendee['email'] == $_attendee['email']) {
                             $new['attendees'][$i] = $_attendee;
                             if ($_attendee['status'] == 'DELEGATED' && ($email = $_attendee['delegated-to'])) {
                                 $delegates[] = strtolower($email);
                             }
 
                             break;
                         }
                     }
                 }
             }
 
             // make sure delegated attendee is not lost
             foreach ($delegates as $delegatee) {
                 if (!in_array($delegatee, $attendees)) {
                     foreach ((array) $old['attendees'] as $attendee) {
                         if ($attendee['email'] && ($email = strtolower($attendee['email'])) && $email == $delegatee) {
                             $new['attendees'][] = $attendee;
                             break;
                         }
                     }
                 }
             }
         }
 
         // We also make sure that status of any attendee
         // is not overriden by NEEDS-ACTION if it was already set
         // which could happen if you work with shared events
         foreach ((array) $new['attendees'] as $i => $attendee) {
             if ($attendee['email'] && $attendee['status'] == 'NEEDS-ACTION') {
                 foreach ($old['attendees'] as $_attendee) {
                     if ($attendee['email'] == $_attendee['email']) {
                         $new['attendees'][$i]['status'] = $_attendee['status'];
                         unset($new['attendees'][$i]['rsvp']);
                         break;
                     }
                 }
             }
         }
     }
 
 
     /*********  Static utility functions  *********/
 
     /**
      * Convert the internal structured data into a vcalendar rrule 2.0 string
      */
     public static function to_rrule($recurrence, $allday = false)
     {
         if (is_string($recurrence)) {
             return $recurrence;
         }
 
         $rrule = '';
         foreach ((array)$recurrence as $k => $val) {
             $k = strtoupper($k);
             switch ($k) {
             case 'UNTIL':
                 // convert to UTC according to RFC 5545
                 if (is_a($val, 'DateTime')) {
                     if (!$allday && empty($val->_dateonly)) {
                         $until = clone $val;
                         $until->setTimezone(new DateTimeZone('UTC'));
                         $val = $until->format('Ymd\THis\Z');
                     }
                     else {
                         $val = $val->format('Ymd');
                     }
                 }
                 break;
             case 'RDATE':
             case 'EXDATE':
                 foreach ((array)$val as $i => $ex) {
                     if (is_a($ex, 'DateTime')) {
                         $val[$i] = $ex->format('Ymd\THis');
                     }
                 }
                 $val = join(',', (array)$val);
                 break;
             case 'EXCEPTIONS':
                 continue 2;
             }
 
             if (strlen($val)) {
                 $rrule .= $k . '=' . $val . ';';
             }
         }
 
         return rtrim($rrule, ';');
     }
 
     /**
      * Convert from fullcalendar date format to PHP date() format string
      */
     public static function to_php_date_format($from)
     {
         // "dd.MM.yyyy HH:mm:ss" => "d.m.Y H:i:s"
         return strtr(strtr($from, array(
             'YYYY' => 'Y',
             'YY'   => 'y',
             'yyyy' => 'Y',
             'yy'   => 'y',
             'MMMM' => 'F',
             'MMM'  => 'M',
             'MM'   => 'm',
             'M'    => 'n',
             'dddd' => 'l',
             'ddd'  => 'D',
             'DD'   => 'd',
             'D'    => 'j',
             'HH'   => '**',
             'hh'   => '%%',
             'H'    => 'G',
             'h'    => 'g',
             'mm'   => 'i',
             'ss'   => 's',
             'TT'   => 'A',
             'tt'   => 'a',
             'T'    => 'A',
             't'    => 'a',
             'u'    => 'c',
         )), array(
             '**'   => 'H',
             '%%'   => 'h',
         ));
     }
 
     /**
      * Convert from PHP date() format to fullcalendar (MomentJS) format string
      */
     public static function from_php_date_format($from)
     {
         // "d.m.Y H:i:s" => "dd.MM.yyyy HH:mm:ss"
         return strtr($from, array(
             'y' => 'YY',
             'Y' => 'YYYY',
             'M' => 'MMM',
             'F' => 'MMMM',
             'm' => 'MM',
             'n' => 'M',
             'j' => 'D',
             'd' => 'DD',
             'D' => 'ddd',
             'l' => 'dddd',
             'H' => 'HH',
             'h' => 'hh',
             'G' => 'H',
             'g' => 'h',
             'i' => 'mm',
             's' => 'ss',
             'c' => '',
         ));
     }
 }
diff --git a/plugins/libcalendaring/tests/RecurrenceTest.php b/plugins/libcalendaring/tests/RecurrenceTest.php
index d17257a8..39c2edd8 100644
--- a/plugins/libcalendaring/tests/RecurrenceTest.php
+++ b/plugins/libcalendaring/tests/RecurrenceTest.php
@@ -1,337 +1,433 @@
 <?php
 
 /**
  * libcalendaring_recurrence tests
  *
  * @author Aleksander Machniak <machniak@apheleia-it.ch>
  *
  * Copyright (C) 2022, Apheleia IT AG <contact@apheleia-it.ch>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
  * published by the Free Software Foundation, either version 3 of the
  * License, or (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU Affero General Public License for more details.
  *
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
 class RecurrenceTest extends PHPUnit\Framework\TestCase
 {
     private $plugin;
 
     function setUp(): void
     {
         $rcube = rcmail::get_instance();
         $rcube->plugins->load_plugin('libcalendaring', true, true);
 
         $this->plugin = $rcube->plugins->get_plugin('libcalendaring');
     }
 
     /**
      * Data for test_end()
      */
     function data_end()
     {
         return [
             // non-recurring
             [
                 [
                     'recurrence' => [],
                     'start' => new DateTime('2017-08-31 11:00:00')
                 ],
                 '2017-08-31 11:00:00', // expected result
             ],
             // daily
             [
                 [
                     'recurrence' => ['FREQ' => 'DAILY', 'INTERVAL' => '1', 'COUNT' => 2],
                     'start' => new DateTime('2017-08-31 11:00:00')
                 ],
                 '2017-09-01 11:00:00',
             ],
             // weekly
             [
                 [
                     'recurrence' => ['FREQ' => 'WEEKLY', 'COUNT' => 3],
                     'start' => new DateTime('2017-08-31 11:00:00'), // Thursday
                 ],
                 '2017-09-14 11:00:00',
             ],
             // UNTIL
             [
                 [
                     'recurrence' => ['FREQ' => 'WEEKLY', 'COUNT' => 3, 'UNTIL' => new DateTime('2017-09-07 11:00:00')],
                     'start' => new DateTime('2017-08-31 11:00:00'), // Thursday
                 ],
                 '2017-09-07 11:00:00',
             ],
             // Infinite recurrence, no count, no until
             [
                 [
                     'recurrence' => ['FREQ' => 'WEEKLY', 'INTERVAL' => '1'],
                     'start' => new DateTime('2017-08-31 11:00:00'), // Thursday
                 ],
                 '2084-09-21 11:00:00',
             ],
 
             // TODO: Test an event with EXDATE/RDATEs
         ];
     }
     /**
      * Data for test_first_occurrence()
      */
     function data_first_occurrence()
     {
         // TODO: BYYEARDAY, BYWEEKNO, BYSETPOS, WKST
 
         return array(
             // non-recurring
             array(
                 array(),                                     // recurrence data
                 '2017-08-31 11:00:00',                       // start date
                 '2017-08-31 11:00:00',                       // expected result
             ),
             // daily
             array(
                 array('FREQ' => 'DAILY', 'INTERVAL' => '1'), // recurrence data
                 '2017-08-31 11:00:00',                       // start date
                 '2017-08-31 11:00:00',                       // expected result
             ),
             // TODO: this one is not supported by the Calendar UI
 /*
             array(
                 array('FREQ' => 'DAILY', 'INTERVAL' => '1', 'BYMONTH' => 1),
                 '2017-08-31 11:00:00',
                 '2018-01-01 11:00:00',
             ),
 */
             // weekly
             array(
                 array('FREQ' => 'WEEKLY', 'INTERVAL' => '1'),
                 '2017-08-31 11:00:00', // Thursday
                 '2017-08-31 11:00:00',
             ),
             array(
                 array('FREQ' => 'WEEKLY', 'INTERVAL' => '1', 'BYDAY' => 'WE'),
                 '2017-08-31 11:00:00', // Thursday
                 '2017-09-06 11:00:00',
             ),
             array(
                 array('FREQ' => 'WEEKLY', 'INTERVAL' => '1', 'BYDAY' => 'TH'),
                 '2017-08-31 11:00:00', // Thursday
                 '2017-08-31 11:00:00',
             ),
             array(
                 array('FREQ' => 'WEEKLY', 'INTERVAL' => '1', 'BYDAY' => 'FR'),
                 '2017-08-31 11:00:00', // Thursday
                 '2017-09-01 11:00:00',
             ),
             array(
                 array('FREQ' => 'WEEKLY', 'INTERVAL' => '2'),
                 '2017-08-31 11:00:00', // Thursday
                 '2017-08-31 11:00:00',
             ),
             array(
                 array('FREQ' => 'WEEKLY', 'INTERVAL' => '3', 'BYDAY' => 'WE'),
                 '2017-08-31 11:00:00', // Thursday
                 '2017-09-20 11:00:00',
             ),
             array(
                 array('FREQ' => 'WEEKLY', 'INTERVAL' => '1', 'BYDAY' => 'WE', 'COUNT' => 1),
                 '2017-08-31 11:00:00', // Thursday
                 '2017-09-06 11:00:00',
             ),
             array(
                 array('FREQ' => 'WEEKLY', 'INTERVAL' => '1', 'BYDAY' => 'WE', 'UNTIL' => '2017-09-01'),
                 '2017-08-31 11:00:00', // Thursday
                 '',
             ),
             // monthly
             array(
                 array('FREQ' => 'MONTHLY', 'INTERVAL' => '1'),
                 '2017-09-08 11:00:00',
                 '2017-09-08 11:00:00',
             ),
-/*
             array(
                 array('FREQ' => 'MONTHLY', 'INTERVAL' => '1', 'BYMONTHDAY' => '8,9'),
                 '2017-08-31 11:00:00',
                 '2017-09-08 11:00:00',
             ),
-*/
             array(
                 array('FREQ' => 'MONTHLY', 'INTERVAL' => '1', 'BYMONTHDAY' => '8,9'),
                 '2017-09-08 11:00:00',
                 '2017-09-08 11:00:00',
             ),
             array(
                 array('FREQ' => 'MONTHLY', 'INTERVAL' => '1', 'BYDAY' => '1WE'),
                 '2017-08-16 11:00:00',
                 '2017-09-06 11:00:00',
             ),
             array(
                 array('FREQ' => 'MONTHLY', 'INTERVAL' => '1', 'BYDAY' => '-1WE'),
                 '2017-08-16 11:00:00',
                 '2017-08-30 11:00:00',
             ),
             array(
                 array('FREQ' => 'MONTHLY', 'INTERVAL' => '2'),
                 '2017-09-08 11:00:00',
                 '2017-09-08 11:00:00',
             ),
-/*
             array(
                 array('FREQ' => 'MONTHLY', 'INTERVAL' => '2', 'BYMONTHDAY' => '8'),
                 '2017-08-31 11:00:00',
                 '2017-09-08 11:00:00', // ??????
             ),
-*/
             // yearly
             array(
                 array('FREQ' => 'YEARLY', 'INTERVAL' => '1'),
                 '2017-08-16 12:00:00',
                 '2017-08-16 12:00:00',
             ),
             array(
                 array('FREQ' => 'YEARLY', 'INTERVAL' => '1', 'BYMONTH' => '8'),
                 '2017-08-16 12:00:00',
                 '2017-08-16 12:00:00',
             ),
 /*
+            // Not supported by Sabre (requires BYMONTH too)
             array(
                 array('FREQ' => 'YEARLY', 'INTERVAL' => '1', 'BYDAY' => '-1MO'),
                 '2017-08-16 11:00:00',
                 '2017-12-25 11:00:00',
             ),
 */
             array(
                 array('FREQ' => 'YEARLY', 'INTERVAL' => '1', 'BYMONTH' => '8', 'BYDAY' => '-1MO'),
                 '2017-08-16 11:00:00',
                 '2017-08-28 11:00:00',
             ),
-/*
             array(
                 array('FREQ' => 'YEARLY', 'INTERVAL' => '1', 'BYMONTH' => '1', 'BYDAY' => '1MO'),
                 '2017-08-16 11:00:00',
                 '2018-01-01 11:00:00',
             ),
             array(
                 array('FREQ' => 'YEARLY', 'INTERVAL' => '1', 'BYMONTH' => '1,9', 'BYDAY' => '1MO'),
                 '2017-08-16 11:00:00',
                 '2017-09-04 11:00:00',
             ),
-*/
             array(
                 array('FREQ' => 'YEARLY', 'INTERVAL' => '2'),
                 '2017-08-16 11:00:00',
                 '2017-08-16 11:00:00',
             ),
             array(
                 array('FREQ' => 'YEARLY', 'INTERVAL' => '2', 'BYMONTH' => '8'),
                 '2017-08-16 11:00:00',
                 '2017-08-16 11:00:00',
             ),
 /*
+            // Not supported by Sabre (requires BYMONTH too)
             array(
                 array('FREQ' => 'YEARLY', 'INTERVAL' => '2', 'BYDAY' => '-1MO'),
                 '2017-08-16 11:00:00',
                 '2017-12-25 11:00:00',
             ),
 */
             // on dates (FIXME: do we really expect the first occurrence to be on the start date?)
             array(
                 array('RDATE' => array(new DateTime('2017-08-10 11:00:00 Europe/Warsaw'))),
                 '2017-08-01 11:00:00',
                 '2017-08-01 11:00:00',
             ),
         );
     }
 
     /**
      * Test for libcalendaring_recurrence::end()
      *
      * @dataProvider data_end
      */
     function test_end($event, $expected)
     {
         $recurrence = new libcalendaring_recurrence($this->plugin, $event);
 
         $end = $recurrence->end();
 
         $this->assertSame($expected, $end ? $end->format('Y-m-d H:i:s') : $end);
     }
 
     /**
      * Test for libcalendaring_recurrence::first_occurrence()
      *
      * @dataProvider data_first_occurrence
      */
     function test_first_occurrence($recurrence_data, $start, $expected)
     {
         $start = new DateTime($start);
         if (!empty($recurrence_data['UNTIL'])) {
             $recurrence_data['UNTIL'] = new DateTime($recurrence_data['UNTIL']);
         }
 
         $recurrence = $this->plugin->get_recurrence();
 
         $recurrence->init($recurrence_data, $start);
         $first = $recurrence->first_occurrence();
 
         $this->assertEquals($expected, $first ? $first->format('Y-m-d H:i:s') : '');
     }
 
     /**
      * Test for libcalendaring_recurrence::first_occurrence() for all-day events
      *
      * @dataProvider data_first_occurrence
      */
     function test_first_occurrence_allday($recurrence_data, $start, $expected)
     {
         $start = new libcalendaring_datetime($start);
         $start->_dateonly = true;
 
         if (!empty($recurrence_data['UNTIL'])) {
             $recurrence_data['UNTIL'] = new DateTime($recurrence_data['UNTIL']);
         }
 
         $recurrence = $this->plugin->get_recurrence();
 
         $recurrence->init($recurrence_data, $start);
         $first = $recurrence->first_occurrence();
 
         $this->assertEquals($expected, $first ? $first->format('Y-m-d H:i:s') : '');
 
         if ($expected) {
             $this->assertTrue($first->_dateonly);
         }
     }
 
     /**
      * Test for libcalendaring_recurrence::next_instance()
      */
     function test_next_instance()
     {
         date_default_timezone_set('America/New_York');
 
         $start = new libcalendaring_datetime('2017-08-31 11:00:00', new DateTimeZone('Europe/Berlin'));
         $event = [
             'start'      => $start,
             'recurrence' => ['FREQ' => 'WEEKLY', 'INTERVAL' => '1'],
             'allday'     => true,
         ];
 
         $recurrence = new libcalendaring_recurrence($this->plugin, $event);
         $next       = $recurrence->next_instance();
 
         $this->assertEquals($start->format('2017-09-07 H:i:s'), $next['start']->format('Y-m-d H:i:s'), 'Same time');
         $this->assertEquals($start->getTimezone()->getName(), $next['start']->getTimezone()->getName(), 'Same timezone');
         $this->assertTrue($next['start']->_dateonly, '_dateonly flag');
     }
+
+    /**
+     * Test for libcalendaring_recurrence::next_instance()
+     */
+    function test_next_instance_exdate()
+    {
+        date_default_timezone_set('America/New_York');
+
+        $start = new libcalendaring_datetime('2023-01-18 10:00:00', new DateTimeZone('Europe/Berlin'));
+        $end = new libcalendaring_datetime('2023-01-18 10:30:00', new DateTimeZone('Europe/Berlin'));
+        $event = [
+            'start' => $start,
+            'end' => $end,
+            'recurrence' => [
+                'FREQ' => 'DAILY',
+                'INTERVAL' => '1',
+                'EXDATE' => [
+                    // Exclude the start date
+                    new libcalendaring_datetime('2023-01-18 10:00:00', new DateTimeZone('Europe/Berlin')),
+                ],
+            ],
+        ];
+
+        $recurrence = new libcalendaring_recurrence($this->plugin, $event);
+
+        $next = $recurrence->next_instance();
+
+        $this->assertEquals('2023-01-19 10:00:00', $next['start']->format('Y-m-d H:i:s'));
+        $this->assertEquals('Europe/Berlin', $next['start']->getTimezone()->getName());
+        $this->assertFalse($next['start']->_dateonly);
+
+        $next = $recurrence->next_instance();
+
+        $this->assertEquals('2023-01-20 10:00:00', $next['start']->format('Y-m-d H:i:s'));
+        $this->assertEquals('Europe/Berlin', $next['start']->getTimezone()->getName());
+        $this->assertFalse($next['start']->_dateonly);
+    }
+
+    /**
+     * Test for libcalendaring_recurrence::next_instance()
+     */
+    function test_next_instance_dst()
+    {
+        date_default_timezone_set('America/New_York');
+
+        $start = new libcalendaring_datetime('2021-03-10 10:00:00', new DateTimeZone('Europe/Berlin'));
+        $end = new libcalendaring_datetime('2021-03-10 10:30:00', new DateTimeZone('Europe/Berlin'));
+        $event = [
+            'start' => $start,
+            'end' => $end,
+            'recurrence' => [
+                'FREQ' => 'MONTHLY',
+                'INTERVAL' => '1',
+            ],
+        ];
+
+        $recurrence = new libcalendaring_recurrence($this->plugin, $event);
+
+        $next = $recurrence->next_instance();
+
+        $this->assertEquals('2021-04-10 10:00:00', $next['start']->format('Y-m-d H:i:s'));
+        $this->assertEquals('Europe/Berlin', $next['start']->getTimezone()->getName());
+
+        $next = $recurrence->next_instance();
+
+        $this->assertEquals('2021-05-10 10:00:00', $next['start']->format('Y-m-d H:i:s'));
+        $this->assertEquals('Europe/Berlin', $next['start']->getTimezone()->getName());
+
+        $start = new libcalendaring_datetime('2021-10-10 10:00:00', new DateTimeZone('Europe/Berlin'));
+        $end = new libcalendaring_datetime('2021-10-10 10:30:00', new DateTimeZone('Europe/Berlin'));
+        $event = [
+            'start' => $start,
+            'end' => $end,
+            'recurrence' => [
+                'FREQ' => 'MONTHLY',
+                'INTERVAL' => '1',
+            ],
+        ];
+
+        $recurrence = new libcalendaring_recurrence($this->plugin, $event);
+
+        $next = $recurrence->next_instance();
+
+        $this->assertEquals('2021-11-10 10:00:00', $next['start']->format('Y-m-d H:i:s'));
+        $this->assertEquals('Europe/Berlin', $next['start']->getTimezone()->getName());
+
+        $next = $recurrence->next_instance();
+
+        $this->assertEquals('2021-12-10 10:00:00', $next['start']->format('Y-m-d H:i:s'));
+        $this->assertEquals('Europe/Berlin', $next['start']->getTimezone()->getName());
+
+        $next = $recurrence->next_instance();
+        $next = $recurrence->next_instance();
+        $next = $recurrence->next_instance();
+        $next = $recurrence->next_instance();
+
+        $this->assertEquals('2022-04-10 10:00:00', $next['start']->format('Y-m-d H:i:s'));
+        $this->assertEquals('Europe/Berlin', $next['start']->getTimezone()->getName());
+
+    }
 }