This issue involves recurring events created on the device which occur in the GMT shadow and are set for relative monthly dates (example: last Tuesday of each month).
Recurring events with these characteristics may not always appear on the date intended on the Zimbra server. The inconsistency seems to be due to the fact that months may have 4 or 5 of a certain day of the week. For example, some months may have four Tuesdays, others may have five. The recurring event occurring in the GMT shadow may show up on the fourth Tuesday in a month with five Tuesdays on the Zimbra server, but still appear correctly on the device.
For time zones west of GMT (Prime Meridian or Longitude Zero), GMT shadow is defined as the hours of the day after GMT midnight - for example, after 7:00 p.m. EST.
What is GMT and why is it used? http://wwp.greenwichmeantime.com/what-is-gmt.htm