Corrupt (?) contact data

I've noticed in my iPhone calendar that there are a small number of birthdays showing on the wrong dates. Checking in Contacts where the information is coming from, the date is incorrect there as well but I can't change it. Entering edit mode and tapping the date crashes Contacts. Exactly the same process for dates which are correct, or entries without dates allows me to set the date this way.

All this data has been entered in Address Book on my Mac and synced across. The problem dates are all correct in Address Book.

I have tried:
- removing the contact from the group and syncing to remove it from the iPhone and then putting it back in.
- changing the date in Address Book and syncing (this does not change the date in Contacts)
- deleting the birthday information altogether from Address Book and syncing (this does not remove the date from Contacts)
- all of the above again, powering off the iPhone between steps in an attempt to clear any cache there may be.

Nothing seems to work. I can successfully remove the person from Contacts but if it is in there the birthday will not change.

Searching these forums and the internet in general I have found discussions of dates being one day out (time zone issues?) but the incorrect dates I am seeing are weeks out.

Has anyone come across this problem and been able to solve it?

I'm syncing with iTunes.
i0S 6.0.1, Mac OS X 10.6.8. iTunes 10.7 (21)

Okay, I've solved the problem after two days of trying and so many syncs I've lost count. I just don't know why what I did I solved the problem.

Here's what I did:
Created a new entry in Address Book in the same group.
Added the surname and initial (to be easily distinguished from full name in the original record), phone numbers, email address and birthday from the original record but ignored the other data.
Synced

At this point what I expected to see was a duplication in Contacts on the iPhone which mirrored the two records in Address Book. The record with the initial did not appear. I assume the Contacts app is doing some automatic de-duping by comparing a number of fields. The original record was still there, with all the information and the correct birthday!

Wondering if it was doing some sort of on the fly merging I deleted the incomplete record in Address Book and synced again. No change. The birthday is now correct in Contacts and editable on the iPhone.

My original idea had been to replace the contact with a newly created one after checking there wasn't some sort of problem with the specific date of birth but it seems this was enough to fix the original problem.