Please use
CodeTags
And I may quick in first reply, but I understood this wouldn't solve your problem, because I suspect validwhen doesn't support "date" fields only
string and primitive data type, still I correct an error for you
Try this code. Also if you want this validwhen approach, then you have break up the date field in three text boxes, like
[day] [month] [year] and then need to check the larger values in end date.
If it still fails then you have to write one plugbale validator for your own requirements, example:
http://raibledesigns.com/rd/date/20030226