This is another calendar sync bug or maybe it’s another ‘no end date’ bug – this time All day events that recur yearly are changed to recur every 12 years. It seems to be related to ActiveSync, Outlook 2007 SP2 and all day events with no end date, possibly affecting only users who sync their mobile device with two computers.

If your calendar is affected by this bug, set the events to have an end date. This reportedly fixes the problem.

Even if you don’t sync with devices, I recommend setting an end date on all recurring appointments and events. If the appointment occurs frequently, set the end date for Dec 31 and make new events each Jan 1. Use 2012 as the end year for less frequent events such as birthdays or set them to end after 10 events. As long as you do not add notes to individual occurrences within the series or create exceptions, you can change the end date later without issue.