Time Zone Changes for 2007 and ZCS
2007 brings changes to time zone definitions for several countries, including the United States, Canada, Bermuda, and Australia. When an appointment is created it includes the dates to adjust for daylight savings and the offset from GMT. Appointments created with 2006's daylight savings transition dates will be incorrect for display of appointments in 2007 during the time where the offset from GMT is different for the two years.
Let's take the United States as an example. In 2007 daylight savings time starts 3 weeks earlier and ends 1 week later than it did in 2006. During these 4 weeks, recurring appointments created with the 2006 rules will be off by 1 hour. Also, single-instance appointments created using the 2006 rules for these 4 weeks will be off by an hour. Appointments that occur in the other 48 weeks of the year (including recurring appointments) are not impacted by the change since the 2006 and 2007 rules are the same for these weeks. A lot of the following text states "the appointment will display incorrectly". The incorrect display will occur only during these 4 weeks.
The 4.5.2 release addresses these changes and also provides a server-based migration utility to change appointments created with pre-2007 time zone rules to 2007 rules.
Microsoft has provided an excellent article  describing the changes and how they impact its OS and products. We recommend all admins read it.
Before we go into the ZCS time zone behavior in 4.5.2, we should look at what the OS vendors have done to address these issues.
- Microsoft has issued a patch for Windows XP relating to KB article 931836. This patch updates OS time zone definitions for 2007. The patch does not change existing appointments in either the ZCO or the Zimbra Server. Microsoft has made this a mandatory patch, but the desktop must be configured to automatically download and install the patch for the user to get it with no other action.
- Windows Vista shipped with the 2007 time zone definitions. This means that it is unnecessary to patch it.
- Apple included 2007 time zone definitions in OS X 10.4.6.
- Novell/SUSE and Red Hat have both provided patches for a variety of their offerings. IBM has produced an overview of Linux time zone issues that is well done and includes links for Linux fixes.[http://www-1.ibm.com/support/docview.wss?rs=0