I don't want to open an another thread, but I'm in a similar situation.
I've created a new usergroup called Birthday, so each user celebrating his birthday is promoted for one day.
The issue is: the promotion only works if the user logs into the forum on hist birthday, otherwise I have to run "Rebuild User Group Promotions" via cron.
The promotion adds a banner to the username, that's the only thing that this promotion does.
Most people don't have time for the forum on their birthday (family/birthday trip/etc), but the rest of the forum can write some "Birthday wishes" on his profile page, dedicated thread, etc, after noticing the "birthday banner".