diff --git a/app/model/kolabobject.py b/app/model/kolabobject.py
index 250c317..e948382 100644
--- a/app/model/kolabobject.py
+++ b/app/model/kolabobject.py
@@ -1,369 +1,374 @@
 # -*- coding: utf-8 -*-
 #
 # Copyright 2014 Kolab Systems AG (http://www.kolabsys.com)
 #
 # Thomas Bruederli <bruederli at 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/>.
 #
 
 import json
 import pytz
 import hashlib
 import datetime
 import logging
 from dateutil.parser import parse as parse_date
 from pykolab.xml.utils import compute_diff
 from collections import OrderedDict
 from email import message_from_string
 from app import storage
 
 log = logging.getLogger('model')
 
 class KolabObject(object):
     """
         Base Model class for accessing Kolab Groupware Object data
     """
     folder_type = 'unknown'
     x_kolab_type = 'application/x-vnd.kolab.*'
 
     def __init__(self, env={}):
         from flask import current_app
 
         self.env = env
         self.config = current_app.config
         self.storage = storage.factory()
 
     def created(self, uid, mailbox, msguid=None):
         """
             Provide created date and user
         """
         changelog = self._object_changelog(uid, mailbox, msguid, 1)
         if changelog and len(changelog) > 0:
             for change in changelog:
                 if change['op'] == 'APPEND':
                     change['uid'] = uid
                     change.pop('op', None)
                     return change
 
         return False
 
     def lastmodified(self, uid, mailbox, msguid=None):
         """
             Provide last change information
         """
         changelog = self._object_changelog(uid, mailbox, msguid, -3)
         if changelog and len(changelog) > 0:
             for change in changelog:
                 if change['op'] == 'APPEND':
                     change['uid'] = uid
                     change.pop('op', None)
                     return change
 
         return False
 
     def changelog(self, uid, mailbox, msguid=None):
         """
             Full changelog
         """
         changelog = self._object_changelog(uid, mailbox, msguid)
         if changelog:
             return dict(uid=uid, changes=changelog)
 
         return False
 
     def get(self, uid, rev, mailbox, msguid=None):
         """
             Retrieve an old revision
         """
         obj = self._get(uid, mailbox, msguid, rev)
 
         if obj is not None:
             return dict(uid=uid, rev=rev, xml=str(obj), mailbox=mailbox)
 
         return False
 
     def _get(self, uid, mailbox, msguid, rev):
         """
             Get an old revision and return the pykolab.xml object
         """
         obj = False
 
         rec = self.storage.get_revision(uid, self._resolve_mailbox_uri(mailbox), msguid, rev)
 
         if rec is not None:
             raw = self.storage.get_message_data(rec)
             try:
                 message = message_from_string(raw.encode('utf8','replace'))
                 obj = self._object_from_message(message) or False
             except Exception, e:
                 log.warning("Failed to parse mime message for UID %s @%s: %r", uid, rev, e)
 
             if obj is False:
                 log.warning("Failed to parse mime message for UID %s @%s", uid, rev)
 
         return obj
 
     def diff(self, uid, rev1, rev2, mailbox, msguid=None, instance=None):
         """
             Compare two revisions of an object and return a list of property changes
         """
         rev_old = rev1
         rev_new = rev2
 
         if rev_old >= rev_new:
             raise ValueError("Invalid argument 'rev'")
 
         old = self._get(uid, mailbox, msguid, rev_old)
         if old == False:
             raise ValueError("Object %s @rev:%s not found" % (uid, str(rev_old)))
 
         new = self._get(uid, mailbox, msguid, rev_new)
         if new == False:
             raise ValueError("Object %s @rev:%s not found" % (uid, str(rev_new)))
 
         # compute diff for the requested recurrence instance
         if instance is not None and hasattr(old, 'get_instance') and hasattr(new, 'get_instance'):
             log.debug("Get recurrence instance %s for object %s", instance, uid)
 
             try:
                 recurrence_date = datetime.datetime.strptime(str(instance), "%Y%m%dT%H%M%S")
             except:
                 try:
                     recurrence_date = datetime.date.strptime(str(instance), "%Y%m%d").date()
                 except:
                     raise ValueError("Invalid isntance identifier %r" % (instance))
 
             old = old.get_instance(recurrence_date)
             if old == None:
                 raise ValueError("Object instance %s-%s @rev:%s not found" % (uid, instance, str(rev_old)))
 
             new = new.get_instance(recurrence_date)
             if new == None:
                 raise ValueError("Object instance %s-%s @rev:%s not found" % (uid, instance, str(rev_new)))
 
-        return dict(uid=uid, rev=rev_new, changes=convert2primitives(compute_diff(old.to_dict(), new.to_dict(), False)))
+        result = dict(uid=uid, rev=rev_new, changes=convert2primitives(compute_diff(old.to_dict(), new.to_dict(), False)))
+
+        if instance is not None:
+            result['instance'] = instance
+
+        return result
 
     def rawdata(self, uid, mailbox, rev, msguid=None):
         """
             Get the full message payload of an old revision
         """
         rec = self.storage.get_revision(uid, self._resolve_mailbox_uri(mailbox), msguid, rev)
         if rec is not None:
             return self.storage.get_message_data(rec)
 
         return False
 
     def _object_from_message(self, message):
         """
              To be implemented in derived classes
         """
         return None
 
     def _object_changelog(self, uid, mailbox, msguid, limit=None):
         """
             Query storage for changelog events related to the given UID
         """
         # this requires a user context
         if not self.env.has_key('REQUEST_USER') or not self.env['REQUEST_USER']:
             return None
 
         # fetch event log from storage
         eventlog = self.storage.get_events(uid, self._resolve_mailbox_uri(mailbox), msguid, limit)
 
         # convert logstash entries into a sane changelog
         event_op_map = {
             'MessageNew': 'APPEND',
             'MessageAppend': 'APPEND',
             'MessageTrash': 'DELETE',
             'MessageMove': 'MOVE',
         }
         last_append_uid = 0
         result = []
 
         if eventlog is not None:
             for log in eventlog:
                 # filter MessageTrash following a MessageAppend event (which is an update operation)
                 if log['event'] == 'MessageTrash' and last_append_uid > int(log['uidset']):
                     continue
 
                 # remember last appended message uid
                 if log['event'] == 'MessageAppend' and log.has_key('uidset'):
                     last_append_uid = int(log['uidset'])
 
                 # compose log entry to return
                 logentry = {
                     'rev': log.get('revision', None),
                     'op': event_op_map.get(log['event'], 'UNKNOWN'),
                     'mailbox': self._convert_mailbox_uri(log.get('mailbox', None))
                 }
                 try:
                     timestamp = parse_date(log['timestamp'])
                     logentry['date'] = datetime.datetime.strftime(timestamp, "%Y-%m-%dT%H:%M:%SZ")
                 except:
                     logentry['date'] = log['timestamp']
 
                 # TODO: translate mailbox identifier back to a relative folder path?
 
                 logentry['user'] = self._get_user_info(log)
 
                 result.append(logentry)
 
         return result
 
     def _resolve_username(self, user):
         """
             Resovle the given username to the corresponding nsuniqueid from LDAP
         """
         # find existing entry in our storage backend
         result = self.storage.get_user(username=user)
 
         if result and result.has_key('id'):
             # TODO: cache this lookup in memory?
             return result['id']
 
         # fall-back: return md5 sum of the username to make usernames work as fields/keys in elasticsearch
         return hashlib.md5(user).hexdigest()
 
     def _get_user_info(self, rec):
         """
             Return user information (name, email) related to the given log entry
         """
         if rec.has_key('user_id'):
             # get real user name from rec['user_id']
             user = self.storage.get_user(id=rec['user_id'])
             if user is not None:
                 return "%(cn)s <%(user)s>" % user
 
         if rec.has_key('user'):
             return rec['user']
 
         elif rec['event'] == 'MessageAppend' and rec['headers'].has_key('From'):
             # fallback to message headers
             return rec['headers']['From'][0]
 
         return 'unknown'
 
     def _resolve_mailbox_uri(self, mailbox):
         """
             Convert the given mailbox string into an absolute URI
             regarding the context of the requesting user.
         """
         # this requires a user context
         if not self.env.has_key('REQUEST_USER') or not self.env['REQUEST_USER']:
             return mailbox
 
         if mailbox is None:
             return None
 
         # mailbox already is an absolute path
         if mailbox.startswith('user/') or mailbox.startswith('shared/'):
             return mailbox
 
         domain = ''
         user = self.env['REQUEST_USER']
         if '@' in user:
             (user,_domain) = user.split('@', 1)
             domain = '@' + _domain
 
         owner = user
         path = '/' + mailbox
 
         # TODO: make this configurable or read from IMAP
         shared_prefix = 'Shared Folders/'
         others_prefix = 'Other Users/'
         imap_delimiter = '/'
 
         # case: shared folder
         if mailbox.startswith(shared_prefix):
             return mailbox[len(shared_prefix):] + domain
 
         # case: other users folder
         if mailbox.startswith(others_prefix):
             (owner, subpath) = mailbox[len(others_prefix):].split(imap_delimiter, 1)
             path = imap_delimiter + subpath
 
         if mailbox.upper() == 'INBOX':
             path = ''
 
         # default: personal namespace folder
         return 'user/' + owner + path + domain
 
     def _convert_mailbox_uri(self, mailbox):
         """
             Convert the given absolute mailbox URI into a relative folder
             name regarding the context of the requesting user.
         """
         if mailbox is None:
             return None
 
         # this requires a user context
         request_user = str(self.env.get('REQUEST_USER', '')).lower()
 
         # TODO: make this configurable or read from IMAP
         shared_prefix = 'Shared Folders'
         others_prefix = 'Other Users'
         imap_delimiter = '/'
         domain = ''
 
         if '@' in mailbox:
             (folder,domain) = mailbox.split('@', 1)
         else:
             folder = mailbox
 
         if folder.startswith('user/'):
             parts = folder.split(imap_delimiter, 2)
             if len(parts) > 2:
                 (prefix,user,path) = parts
             else:
                 (prefix,user) = parts
                 path = ''
 
             if len(path) == 0:
                 path = 'INBOX'
 
             if not (user + '@' + domain).lower() == request_user:
                 folder = imap_delimiter.join([others_prefix, user, path])
             else:
                 folder = path
 
         elif folder.startswith('shared/'):
             folder = imap_delimiter.join([shared_prefix, folder])
 
         return folder
 
 
 #####  Utility functions
 
 
 def convert2primitives(struct):
     """
         Convert complex types like datetime into primitives which can be serialized into JSON
     """
     out = None
     if isinstance(struct, datetime.datetime):
         tz = 'Z' if struct.tzinfo == pytz.utc else '%z'
         out = struct.strftime('%Y-%m-%dT%H:%M:%S' + tz)
     elif isinstance(struct, datetime.date):
         out = struct.strftime('%Y-%m-%d')
     elif isinstance(struct, list):
         out = [convert2primitives(x) for x in struct]
     elif isinstance(struct, OrderedDict):
         out = OrderedDict([(key,convert2primitives(struct[key])) for key in struct.keys()])
     elif isinstance(struct, dict):
         out = dict(zip(struct.keys(), map(convert2primitives, struct.values())))
     else:
         out = struct
 
     return out
diff --git a/tests/test_api.py b/tests/test_api.py
index 8f44f4b..0c74e9d 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -1,200 +1,240 @@
 import httplib, json, base64, hmac, hashlib, email
 from twisted.trial import unittest
 
 class TestAPI(unittest.TestCase):
     reqid = 0
 
     # test config
     config = {
         'api_host': '127.0.0.1',
         'api_port': 8080,
         'api_user': 'webclient',
         'api_pass': 'Welcome2KolabSystems',
         'api_secret': '8431f19170e7f90d4107bf4b169baf'
     }
 
     @classmethod
     def setUp(self):
         # TODO: reset riak buckets and fill with sample data
         # TODO: fire up the web service
         pass
 
     def _api_request(self, user, method, sign=True, **kwargs):
         """
             Helper method to send JSON-RPC calls to the API
         """
         headers = {
             'Authorization': 'Basic ' + base64.b64encode(self.config['api_user'] + ':' + self.config['api_pass']),
             'Content-type': "application/json",
             'X-Request-User': user
         }
 
         self.rpcerror = None
         self.reqid += 1
         body = {
             'jsonrpc': '2.0',
             'id': self.reqid,
             'method': method,
             'params': kwargs
         }
         sbody = json.dumps(body)
 
         if sign:
             headers['X-Request-Sign'] = hmac.new(
                 key=self.config['api_secret'],
                 msg=headers.get('X-Request-User') + ':' + sbody,
                 digestmod=hashlib.sha256
             ).hexdigest()
 
         try:
             conn = httplib.HTTPConnection(self.config['api_host'], self.config['api_port'])
             conn.request('POST', '/api/rpc', sbody, headers)
 
             result = None
             response = conn.getresponse()
             if response.status == 200:
                 result = json.loads(response.read())
                 if result.has_key('error'):
                     self.rpcerror = result['error']
                 if result.has_key('result') and result.has_key('id') and result['id'] == self.reqid:
                     return result['result']
 
             # print "JSON-RPC response: %d %s; %r" % (response.status, response.reason, result)
 
         except Exception, e:
             print "JSON-RPC error: %r" % (e)
 
         return False
 
     def assertRPCError(self, code=None):
         self.assertIsInstance(self.rpcerror, dict)
         if code is not None:
             self.assertEqual(self.rpcerror['code'], code)
 
     def test_001_json_rpc(self):
         res = self._api_request('', 'system.keygen', sign=False)
         self.assertFalse(res)
         self.assertRPCError(-32600)
 
         res = self._api_request('', 'system.keygen', sign=True)
         self.assertIsInstance(res, dict)
         self.assertTrue(res.has_key('key'))
         self.assertTrue(res.has_key('expires'))
 
     def test_002_invalid_request(self):
         res = self._api_request('john.doe@example.org', 'event.bogus',
             uid='6EE0570E8CA21DDB67FC9ADE5EE38E7F-XXXXXXXXXXXXXXX'
         )
         self.assertFalse(res)
         self.assertRPCError(-32601)  # Method not found
 
         res = self._api_request('john.doe@example.org', 'event.created',
             uid='6EE0570E8CA21DDB67FC9ADE5EE38E7F-A4BF5BBB9FEAA271'
         )
         self.assertFalse(res)
         self.assertRPCError(-32602)  # Invalid params
 
         res = self._api_request('john.doe@example.org', 'event.created',
             uid='6EE0570E8CA21DDB67FC9ADE5EE38E7F-INVALID-UID',
             mailbox='Calendar'
         )
         self.assertFalse(res)
 
     def test_003_created_lastmodified(self):
         created = self._api_request('john.doe@example.org', 'event.created',
             uid='6EE0570E8CA21DDB67FC9ADE5EE38E7F-A4BF5BBB9FEAA271',
             mailbox='Testing',
             msguid=2
         )
         self.assertIsInstance(created, dict)
         self.assertTrue(created.has_key('date'))
         self.assertTrue(created.has_key('user'))
         self.assertTrue(created.has_key('rev'))
         self.assertIn('john.doe@example.org', created['user'])
 
         changed = self._api_request('john.doe@example.org', 'event.lastmodified',
             uid='6EE0570E8CA21DDB67FC9ADE5EE38E7F-A4BF5BBB9FEAA271',
             mailbox='Testing'
         )
         self.assertIsInstance(changed, dict)
         self.assertTrue(changed.has_key('rev'))
 
         self.assertTrue(changed['rev'] > created['rev'])
         self.assertTrue(changed['date'] > created['date'])
 
     def test_004_changelog(self):
         changelog = self._api_request('john.doe@example.org', 'event.changelog',
             uid='5A637BE7895D785671E1732356E65CC8-A4BF5BBB9FEAA271',
             mailbox='Calendar',
             msguid=22
         )
         # print json.dumps(changelog, indent=4)
         self.assertIsInstance(changelog, dict)
         self.assertTrue(changelog.has_key('changes'))
         self.assertTrue(changelog.has_key('uid'))
         self.assertEqual(len(changelog['changes']), 2)
 
         one = changelog['changes'][0]
         two = changelog['changes'][1]
         self.assertTrue(two['rev'] > one['rev'])
 
     def test_005_get_revision(self):
         changelog = self._api_request('john.doe@example.org', 'event.changelog',
             uid='5A637BE7895D785671E1732356E65CC8-A4BF5BBB9FEAA271',
             mailbox='Calendar'
         )
         self.assertIsInstance(changelog, dict)
         self.assertEqual(len(changelog.get('changes', [])), 2)
 
         rev1 = self._api_request('john.doe@example.org', 'event.get',
             uid='5A637BE7895D785671E1732356E65CC8-A4BF5BBB9FEAA271',
             mailbox='Calendar',
             rev=changelog['changes'][0]['rev']
         )
         self.assertIsInstance(rev1, dict)
         self.assertTrue(rev1.has_key('xml'))
         self.assertNotIn('<location>', rev1['xml'])
 
         rev2 = self._api_request('john.doe@example.org', 'event.get',
             uid='5A637BE7895D785671E1732356E65CC8-A4BF5BBB9FEAA271',
             mailbox='Calendar',
             rev=changelog['changes'][1]['rev']
         )
         self.assertIsInstance(rev2, dict)
         self.assertTrue(rev2.has_key('xml'))
         self.assertIn('<text>Somewhere else</text>', rev2['xml'])
 
         msgdata = self._api_request('john.doe@example.org', 'event.rawdata',
             uid='5A637BE7895D785671E1732356E65CC8-A4BF5BBB9FEAA271',
             mailbox='Calendar',
             rev=changelog['changes'][0]['rev']
         )
         self.assertIsInstance(msgdata, unicode)
 
         message = email.message_from_string(msgdata.encode('utf8','replace'))
         self.assertIsInstance(message, email.message.Message)
         self.assertTrue(message.is_multipart())
 
     def test_006_diff(self):
         changelog = self._api_request('john.doe@example.org', 'event.changelog',
             uid='5A637BE7895D785671E1732356E65CC8-A4BF5BBB9FEAA271',
             mailbox='Calendar'
         )
         self.assertIsInstance(changelog, dict)
         self.assertEqual(len(changelog.get('changes', [])), 2)
         rev1 = changelog['changes'][0]['rev']
         rev2 = changelog['changes'][1]['rev']
 
         diff = self._api_request('john.doe@example.org', 'event.diff',
-            uid='5A637BE7895D785671E1732356E65CC8-A4BF5BBB9FEAA271',
+            uid=changelog['uid'],
             mailbox='Calendar',
             rev1=rev1,
             rev2=rev2
         )
         # print json.dumps(diff, indent=4)
         self.assertIsInstance(diff, dict)
         self.assertTrue(diff.has_key('changes'))
         self.assertTrue(diff.has_key('uid'))
         self.assertEqual(len(diff['changes']), 6)
 
-        
\ No newline at end of file
+    def test_007_diff_instance(self):
+        changelog = self._api_request('john.doe@example.org', 'event.changelog',
+            uid='8B3B2C54C5218FC09EBC840E6289F5E5-A4BF5BBB9FEAA271',
+            mailbox='Calendar'
+        )
+        self.assertIsInstance(changelog, dict)
+        self.assertEqual(len(changelog.get('changes', [])), 2)
+
+        rev1 = changelog['changes'][0]['rev']
+        rev2 = changelog['changes'][1]['rev']
+
+        diff = self._api_request('john.doe@example.org', 'event.diff',
+            uid=changelog['uid'],
+            mailbox='Calendar',
+            rev1=rev1,
+            rev2=rev2,
+            instance='20150324T210000'
+        )
+        self.assertIsInstance(diff, dict)
+        self.assertTrue(diff.has_key('instance'))
+        self.assertEqual(len(diff['changes']), 1)  # no change (except lastmodified-date) in this instance
+
+        diff = self._api_request('john.doe@example.org', 'event.diff',
+            uid=changelog['uid'],
+            mailbox='Calendar',
+            rev1=rev1,
+            rev2=rev2,
+            instance='20150325T210000'
+        )
+        self.assertIsInstance(diff, dict)
+        self.assertEqual(len(diff['changes']), 4) # changes for start,end,sequence,lastmodified-date
+
+        diff = self._api_request('john.doe@example.org', 'event.diff',
+            uid=changelog['uid'],
+            mailbox='Calendar',
+            rev1=rev1,
+            rev2=rev2,
+            instance='20150330T210000'
+        )
+        self.assertFalse(diff)
+        self.assertRPCError(-32603)  # report error for invalid instance