You have a few techniques at your disposal.
1) On the server, use PHP to check the values submitted by the user. Display an error if their values are wrong, and force them to enter it again. This is the most idiot-proof way.
4) Fancy-pants degradable AJAX form validation (example).
In any case, I'd recommend having three separate labeled fields (day, month and year) to prevent mm/dd/yyyy vs. dd/mm/yyyy misunderstandings.