With that sort of format, you can never be absolutely sure. For instance, if the user enters 01/03/07 can you be sure whether he means 01 March 2007, 03 January 2007, or 07 March 2001? As Alimadzi suggests, it's a lot easier to be relatively sure if you give the user 3 select boxes to use rather than counting on him typing it in exactly the way you want it to be.