Here's a thread that mentions it:
http://www.vbulletin.com/forum/forum...r-4-to-be-long
I guess the solution there is to move the affected users to a new user group then back again. I also saw it mentioned that if you edit the user's profile in the admincp and save without changing anything, it will be fixed (although you have to do it for each affected user).
If you look in the database and see what the affected users have in their for their birthday, you might be able to figure out some SQL to fix it.