diff --git a/plugins/libcalendaring/composer.json b/plugins/libcalendaring/composer.json
--- a/plugins/libcalendaring/composer.json
+++ b/plugins/libcalendaring/composer.json
@@ -24,8 +24,8 @@
         }
     ],
     "require": {
-        "php": ">=5.4.0",
+        "php": ">=5.5",
         "roundcube/plugin-installer": ">=0.1.3",
-        "sabre/vobject": "~3.5.3"
+        "sabre/vobject": "~4.3.5"
     }
 }
diff --git a/plugins/libcalendaring/lib/Horde_Date.php b/plugins/libcalendaring/lib/Horde_Date.php
--- a/plugins/libcalendaring/lib/Horde_Date.php
+++ b/plugins/libcalendaring/lib/Horde_Date.php
@@ -1096,7 +1096,7 @@
 
     protected function _initializeFromObject($date)
     {
-        if ($date instanceof DateTime) {
+        if ($date instanceof DateTime || $date instanceof DateTimeImmutable) {
             $this->_year  = (int)$date->format('Y');
             $this->_month = (int)$date->format('m');
             $this->_mday  = (int)$date->format('d');
diff --git a/plugins/libcalendaring/lib/Horde_Date_Recurrence.php b/plugins/libcalendaring/lib/Horde_Date_Recurrence.php
--- a/plugins/libcalendaring/lib/Horde_Date_Recurrence.php
+++ b/plugins/libcalendaring/lib/Horde_Date_Recurrence.php
@@ -1565,7 +1565,7 @@
         // Exceptions.
         if (isset($hash['exclusion'])) {
             foreach ($hash['exclusion'] as $exception) {
-                if ($exception instanceof DateTime) {
+                if ($exception instanceof DateTime || $exception instanceof DateTimeImmutable) {
                     $this->exceptions[] = $exception->format('Ymd');
                 }
             }
@@ -1573,7 +1573,7 @@
 
         if (isset($hash['complete'])) {
             foreach ($hash['complete'] as $completion) {
-                if ($exception instanceof DateTime) {
+                if ($exception instanceof DateTime || $exception instanceof DateTimeImmutable) {
                     $this->completions[] = $completion->format('Ymd');
                 }
             }
diff --git a/plugins/libcalendaring/lib/libcalendaring_itip.php b/plugins/libcalendaring/lib/libcalendaring_itip.php
--- a/plugins/libcalendaring/lib/libcalendaring_itip.php
+++ b/plugins/libcalendaring/lib/libcalendaring_itip.php
@@ -467,7 +467,7 @@
                             if ($key == 'allday') {
                                 $event[$key] = $event[$key] == 'true';
                             }
-                            $value = $existing[$key] instanceof DateTime ? $existing[$key]->format('c') : $existing[$key];
+                            $value = ($existing[$key] instanceof DateTime || $existing[$key] instanceof DateTimeImmutable) ? $existing[$key]->format('c') : $existing[$key];
                             $num++;
                             $got += intval($value == $event[$key]);
                         }
@@ -683,7 +683,7 @@
             // For replies we need more metadata
             foreach (array('start', 'end', 'due', 'allday', 'recurrence', 'location') as $key) {
                 if (isset($event[$key])) {
-                    $metadata[$key] = $event[$key] instanceof DateTime ? $event[$key]->format('c') : $event[$key];
+                    $metadata[$key] = ($event[$key] instanceof DateTime || $event[$key] instanceof DateTimeImmutable) ? $event[$key]->format('c') : $event[$key];
                 }
             }
         }
diff --git a/plugins/libcalendaring/lib/libcalendaring_recurrence.php b/plugins/libcalendaring/lib/libcalendaring_recurrence.php
--- a/plugins/libcalendaring/lib/libcalendaring_recurrence.php
+++ b/plugins/libcalendaring/lib/libcalendaring_recurrence.php
@@ -127,14 +127,14 @@
     public function end()
     {
         // recurrence end date is given
-        if ($this->recurrence['UNTIL'] instanceof DateTime) {
+        if ($this->recurrence['UNTIL'] instanceof DateTime || $this->recurrence['UNTIL'] instanceof DateTimeImmutable) {
             return $this->recurrence['UNTIL'];
         }
 
         // take the last RDATE entry if set
         if (is_array($this->recurrence['RDATE']) && !empty($this->recurrence['RDATE'])) {
             $last = end($this->recurrence['RDATE']);
-            if ($last instanceof DateTime) {
+            if ($last instanceof DateTime || $last instanceof DateTimeImmutable) {
               return $last;
             }
         }
diff --git a/plugins/libcalendaring/libcalendaring.php b/plugins/libcalendaring/libcalendaring.php
--- a/plugins/libcalendaring/libcalendaring.php
+++ b/plugins/libcalendaring/libcalendaring.php
@@ -194,7 +194,7 @@
             $dt = rcube_utils::anytodatetime($dt);
         }
 
-        if ($dt instanceof DateTime && empty($dt->_dateonly) && !$dateonly) {
+        if (($dt instanceof DateTime || $dt instanceof DateTimeImmutable) && empty($dt->_dateonly) && !$dateonly) {
             $dt->setTimezone($this->timezone);
         }
 
@@ -499,7 +499,7 @@
     public static function to_client_alarms($valarms)
     {
         return array_map(function($alarm){
-            if ($alarm['trigger'] instanceof DateTime) {
+            if ($alarm['trigger'] instanceof DateTime || $alarm['trigger'] instanceof DateTimeImmutable) {
                 $alarm['trigger'] = '@' . $alarm['trigger']->format('U');
             }
             else if ($trigger = libcalendaring::parse_alarm_value($alarm['trigger'])) {
@@ -577,7 +577,7 @@
             break;
         }
 
-        if ($trigger instanceof DateTime) {
+        if ($trigger instanceof DateTime || $trigger instanceof DateTimeImmutable) {
             $text .= ' ' . $rcube->gettext(array(
                 'name' => 'libcalendaring.alarmat',
                 'vars' => array('datetime' => $rcube->format_date($trigger))
@@ -657,7 +657,7 @@
         foreach ($rec['valarms'] as $alarm) {
             $notify_time = null;
 
-            if ($alarm['trigger'] instanceof DateTime) {
+            if ($alarm['trigger'] instanceof DateTime || $alarm['trigger'] instanceof DateTimeImmutable) {
                 $notify_time = $alarm['trigger'];
             }
             else if (is_string($alarm['trigger'])) {
@@ -1183,7 +1183,7 @@
      * @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 
+     * @return array Hash array with the parsed iCal
      */
     public function mail_get_itip_object($mbox, $uid, $mime_id, $type = null)
     {
@@ -1309,7 +1309,7 @@
     {
         $instance_date = !empty($event['recurrence_date']) ? $event['recurrence_date'] : $event['start'];
 
-        if ($instance_date instanceof DateTime) {
+        if ($instance_date instanceof DateTime || $instance_date instanceof DateTimeImmutable) {
             // According to RFC5545 (3.8.4.4) RECURRENCE-ID format should
             // be date/date-time depending on the main event type, not the exception
             if ($allday === null) {
diff --git a/plugins/libcalendaring/libvcalendar.php b/plugins/libcalendaring/libvcalendar.php
--- a/plugins/libcalendaring/libvcalendar.php
+++ b/plugins/libcalendaring/libvcalendar.php
@@ -149,7 +149,7 @@
                 }
             }
 
-            $vobject = VObject\Reader::read($vcal, VObject\Reader::OPTION_FORGIVING | VObject\Reader::OPTION_IGNORE_INVALID_LINES);
+            $vobject = Sabre\VObject\Reader::read($vcal, Sabre\VObject\Reader::OPTION_FORGIVING | Sabre\VObject\Reader::OPTION_IGNORE_INVALID_LINES);
             if ($vobject)
                 return $this->import_from_vobject($vobject);
         }
@@ -247,7 +247,7 @@
             }
             catch (Exception $e) {
                 if ($this->forward_exceptions) {
-                    throw new VObject\ParseException($e->getMessage() . " in\n" . $buffer);
+                    throw new Sabre\VObject\ParseException($e->getMessage() . " in\n" . $buffer);
                 }
                 else {
                     // write the failing section to error log
@@ -421,8 +421,8 @@
         }
 
         // map other attributes to internal fields
-        foreach ($ve->children as $prop) {
-            if (!($prop instanceof VObject\Property))
+        foreach ($ve->children() as $prop) {
+            if (!($prop instanceof Sabre\VObject\Property))
                 continue;
 
             $value = strval($prop);
@@ -645,7 +645,7 @@
             $trigger = null;
             $alarm   = array();
 
-            foreach ($valarm->children as $prop) {
+            foreach ($valarm->children() as $prop) {
                 $value = strval($prop);
 
                 switch ($prop->name) {
@@ -713,14 +713,14 @@
         }
 
         // assign current timezone to event start/end
-        if (!empty($event['start']) && $event['start'] instanceof DateTime) {
+        if (!empty($event['start']) && ($event['start'] instanceof DateTime || $event['start'] instanceof DateTimeImmutable)) {
             $this->_apply_timezone($event['start']);
         }
         else {
             unset($event['start']);
         }
 
-        if (!empty($event['end']) && $event['end'] instanceof DateTime) {
+        if (!empty($event['end']) && ($event['end'] instanceof DateTimeImmutable || $event['end'] instanceof DateTimeImmutable)) {
             $this->_apply_timezone($event['end']);
         }
         else {
@@ -740,7 +740,7 @@
 
         // minimal validation
         if (empty($event['uid']) || ($event['_type'] == 'event' && empty($event['start']) != empty($event['end']))) {
-            throw new VObject\ParseException('Object validation failed: missing mandatory object properties');
+            throw new Sabre\VObject\ParseException('Object validation failed: missing mandatory object properties');
         }
 
         return $event;
@@ -775,8 +775,8 @@
         $this->freebusy = array('_type' => 'freebusy', 'periods' => array());
         $seen = array();
 
-        foreach ($ve->children as $prop) {
-            if (!($prop instanceof VObject\Property))
+        foreach ($ve->children() as $prop) {
+            if (!($prop instanceof Sabre\VObject\Property))
                 continue;
 
             $value = strval($prop);
@@ -864,7 +864,7 @@
         if (empty($prop)) {
             return $as_array ? array() : null;
         }
-        else if ($prop instanceof VObject\Property\iCalendar\DateTime) {
+        else if ($prop instanceof Sabre\VObject\Property\iCalendar\DateTime) {
             if (count($prop->getDateTimes()) > 1) {
                 $dt = array();
                 $dateonly = !$prop->hasTime();
@@ -880,7 +880,7 @@
                 }
             }
         }
-        else if ($prop instanceof VObject\Property\iCalendar\Period) {
+        else if ($prop instanceof Sabre\VObject\Property\iCalendar\Period) {
             $dt = array();
             foreach ($prop->getParts() as $val) {
                 try {
@@ -903,7 +903,7 @@
                 }
             }
         }
-        else if ($prop instanceof \DateTime) {
+        else if ($prop instanceof \DateTime || $prop instanceof \DateTimeImmutable) {
             $dt = $prop;
         }
 
@@ -1000,7 +1000,7 @@
         $this->method = $method;
 
         // encapsulate in VCALENDAR container
-        $vcal = new VObject\Component\VCalendar();
+        $vcal = new Sabre\VObject\Component\VCalendar();
         $vcal->VERSION = '2.0';
         $vcal->PRODID = $this->prodid;
         $vcal->CALSCALE = 'GREGORIAN';
@@ -1056,7 +1056,7 @@
     {
         $type = !empty($event['_type']) ? $event['_type'] : 'event';
 
-        $cal = $vcal ?: new VObject\Component\VCalendar();
+        $cal = $vcal ?: new Sabre\VObject\Component\VCalendar();
         $ve = $cal->create($this->type_component_map[$type]);
         $ve->UID = $event['uid'];
 
@@ -1087,7 +1087,7 @@
         }
 
         // we're exporting a recurrence instance only
-        if (!$recurrence_id && !empty($event['recurrence_date']) && $event['recurrence_date'] instanceof DateTime) {
+        if (!$recurrence_id && !empty($event['recurrence_date']) && ($event['recurrence_date'] instanceof DateTime || $event['recurrence_date'] instanceof DateTimeImmutable)) {
             $recurrence_id = $this->datetime_prop($cal, 'RECURRENCE-ID', $event['recurrence_date'], false, !empty($event['allday']));
             if (!empty($event['thisandfuture'])) {
                 $recurrence_id->add('RANGE', 'THISANDFUTURE');
@@ -1129,7 +1129,7 @@
             // add EXDATEs each one per line (for Thunderbird Lightning)
             if (is_array($exdates)) {
                 foreach ($exdates as $exdate) {
-                    if ($exdate instanceof DateTime) {
+                    if ($exdate instanceof DateTime || $exdate instanceof DateTimeImmutable) {
                         $ve->add($this->datetime_prop($cal, 'EXDATE', $exdate));
                     }
                 }
@@ -1194,7 +1194,7 @@
             foreach ($event['valarms'] as $alarm) {
                 $va = $cal->createComponent('VALARM');
                 $va->action = $alarm['action'];
-                if ($alarm['trigger'] instanceof DateTime) {
+                if ($alarm['trigger'] instanceof DateTime || $alarm['trigger'] instanceof DateTimeImmutable) {
                     $va->add($this->datetime_prop($cal, 'TRIGGER', $alarm['trigger'], true, null, true));
                 }
                 else {
@@ -1236,7 +1236,7 @@
             if (!empty($val[3])) {
                 $va->add('TRIGGER', $val[3]);
             }
-            else if ($val[0] instanceof DateTime) {
+            else if ($val[0] instanceof DateTime || $val[0] instanceof DateTimeImmutable) {
                 $va->add($this->datetime_prop($cal, 'TRIGGER', $val[0], true, null, true));
             }
             $ve->add($va);
@@ -1384,7 +1384,7 @@
 
         if (!$from) $from = time();
         if (!$to)   $to = $from;
-        if (!$cal)  $cal = new VObject\Component\VCalendar();
+        if (!$cal)  $cal = new Sabre\VObject\Component\VCalendar();
 
         if (is_string($tzid)) {
             try {
@@ -1512,7 +1512,7 @@
  * Override Sabre\VObject\Property\Text that quotes commas in the location property
  * because Apple clients treat that property as list.
  */
-class vobject_location_property extends VObject\Property\Text
+class vobject_location_property extends Sabre\VObject\Property\Text
 {
     /**
      * List of properties that are considered 'structured'.