I was experimenting with the user group promotions and I'm having a problem with the promotion "Today is user's birthday". Only about 1/4 the users with a birthday today are being moved into the correct group. I've tried running the promotion cron multiple times but it doesn't seem to work. I thought it might have something to do with the user's timezone or birthday privacy settings, but there was no consistency between users that were moved in and those that weren't. Any ideas what could be going on?