Ever since Exchange sync (including active sync) was added to iOS, it has had a bug that can shift a birthday of a contact by one day. This occurs when the iOS device is receiving the birthday via a sync with an Exchange based system. Well it still there in iOS 6. The work around is to manually fix each of the incorrect birthdays in your contacts directly on your iOS device. You can turn off the displaying of incorrect birthdays in the calendar by unchecking the birthdays under other. Here is a little info of where the bug originated (sample code) and how Microsoft has already fixed it in their own clients. https://discussions.apple.com/thread/3245017 So in case you see multiple birthdays or incorrect birthdays in the calendar that is why.