Here is our current findings with OSX Yosemite and Zimbra 8.5p2. The default Calendar.app is not playing well with Zimbra and CalDav.
1) Timezone Handling.
An invite from a Zimbra user in a different timezone on any platform other than Outlook/Windows to a Mac Yosemite/Mavericks Zimbra CalDav user will be added with an incorrect time. The Yosemite/Mavericks notification center will display the correct time but it will be added to Calendar.app with the timezone information stripped. e.g. an invite for 7pm CST will be added as 7pm Local. We have a case open on this one. (this was not a problem in Zimbra 8.0)
2) Create meeting does not forward to external invitees
Add an appointment and add an external invitee - the invite is never sent (this worked in Mavericks)
3) Meeting modification does not forward to external invitees
Modify an appointment and the update is never sent (this worked in Mavericks)
4) "Revert" a meeting deletes the meeting from the Calendar
Previously when modifying a meeting the "Revert" option would undo any changes (this worked in Mavericks).
We realize that some of these problems are probably Yosemite Calendar.app issues only. We think that the the first problem (timezone handling) is logically the format of the ICS file so could be fixed by either Zimbra or by Apple. None of these problems have been seen when using BuyCal for Yosemite but our users who do not have BusyCal unfortunately see this as a Zimbra issue.
How are other Zimbra 8.5 users getting on with Yosemite and Calendar.app ?