put null (empty) value to datetime control

Recently, I have lot trouble with the datetime control. Now, I need to clear up the users invalid input. For example: users enter 07/02/2006 for the date of birth field (DOB). Because the program set 06/30/2006 is the latest date. We need to give use a warring and clear 07/02/2006 in the field. I tried to use ClearSelection() method in C#. But the method somehow does not work with the event OnDateChanged? So I may have to looking for JavaScript? Please see if you can help me out. Many thanks.