In this calendar sync bug All Day events
that recur yearly are changed to recur every 12 years. This bug first
appeared with the release of Office 2007 SP2 and a and
affects devices that sync with Outlook 2007 using ActiveSync. It may
only affect users who sync their device between two different computers.
If your calendar is affected by this bug, set the events to have an end date. This reportedly fixes the problem. A future update may address 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.