http://www.datejs.com/ or http://www.coolite.com/datejs/
This library extends the built-in Date object with useful methods. I used it on Intel Solution Architect 2013 project. See claimform/index.php.
Calling Date.parse('13/5/2013') returns a Date object representing May 13th, 2013.
Calling Date.parse('123') returns a null (because that is not valid date).
Until all browsers are HTML5 ready, the easiest way to drop in a date picker is to use jQuery UI Datepicker.