Humm, a bit late me thinks.
The Exchange Calendar Update Tool (Msextmz.exe) is the tool that Exchange administrators can use to apply time zone changes to a calendar for one or more users. The Exchange Calendar Update Tool uses the Time Zone Data Update Tool for Microsoft Office Outlook (the Outlook Time Zone Data Update tool or Tzmove.exe) to update Exchange calendar information. This document describes how the Exchange Calendar Update Tool works. This article also contains some of the common error results that you may experience when you run it.
This is a bit late A troubleshooting guide to the Exchange Calendar Update Tool (Msextmz.exe)