Similar problem here - see this thread:
http://forum.ppcgeeks.com/showthread.php?t=105165
I sync with WMDC/Outlook 2007, with the Outlook Calendar showing correct entries, but the phone's Calendar messes up. My wife's birthday shows in Outlook Contacts correctly on June 1st. But if I look on my TP2, her Contact Info there shows May 31st, while the Calendar shows her birthday as January 1st. Pretty messed up right here...