If you tt to your b-day twice in one year, it won't do it the second time.
Also, if you changed your birthday on the 3DS settings, then tted to the date you set it as in-game, it won't work since your birthday is the one you registered at the beginning of the game.
I tt to my b-day all the time, but you have to do it once a year, so I am in 2012 right now due to my b-day crazing xD