Another year, another time zone change to make a mess of Outlook's appointments. The US and Canadian governments changed the time daylight savings time begins and ends, with the new dates going into effect this year. For this reason, Windows will need to have the time zone definitions updated, otherwise, for a few weeks each spring and fall, appointments will be off by 1 hour. Once the Windows time zone update is applied, everything should be fine. If you have a lot of appointments, it may take several minutes for Outlook to adjust your appointments (this capability is built into Outlook 2007, an update should be available sometime this month to change appointments in earlier versions). Also, your all day events may show the small clocks that indicate the event has a beginning and end time.

http://www.microsoft.com/windows/timezone/dst2007.mspx has more information and links. Note that anyone using an older version of Windows will need to use a utility to update it themselves but a patch should be automatically installed for Windows XP SP2. Vista is not affected; it ships with the new time zone definitions.

Should you update if you are not in the US or Canada or do not use Outlook?
Yes. This can affect meetings scheduled with people in the affected time zones during the affected weeks in March and November. Anyone in a US or Canadian time zone will have the wrong time on their computer for a few weeks each spring and fall, which can affect network connectivity (and irritates some people because email time stamps are wrong).

More time zone articles: http://www.microsoft.com/windows/timezone.mspx