Javascript Date Validator OnBlur

I have a Classic ASP page with fields. Several of them are dates and I need a way to validate them when the user exits the field with either mouse click elsewhere or a keyboard tab. Acceptable entries would be blank/no input (our code stores a null in the db when this happens), a full U.S. date format ("01/22/2013", "1/22/2013", etc.), a 2 digit year ("1/22/13") or a short entry of 1/22, which should default to 1/22/2013 at this point. If the user inputs something like 22/44/13, it needs to give an alert/message box that that date is not valid, and highlight that field instead of tabbing out or sending the focus to the next screen.