Don't worry, no-one can change the birthday on your account: not unless they either sent a ticket to TNT or could somehow access the database.
Just try again in a while. Either you just made a few mistakes in a row, or the birthday you chose for you account is not your actual birthday.
I know no one can change my birthday, but now I think someone is trying to hack me or something!
I did not make any mistakes and have never entered the wrong b-day, and I swear it I put my actual b-day on my account.