I have seen many people who had issues with date validation (validating a textbox for a date field). Usually they end up writing regular expressions which work fine but there is a much easier way to to it using Visual Studio 2005. (And I am not talking about using the AJAX Control Toolkit)

You can use a 'Compare Validator' and in its behavior attributes provide the textbox name for "ControlToValidate", set the "Operator" to DataTypeCheck and the "Type" to Date. And this should work fine.

Below is the image for this control.