How do i format a date from a user input?

I am writing Javascript for a dynamic stamp date field that requests a users input.

var cEntry = "Enter Date:";

var cTitle = "Item Numbers Date";

if(event.source.forReal && (event.source.stampName == "#g6KmvX-x4BphyE7tMLhbdC"))

{

var cDate = app.response(cEntry, cTitle);

event.value = cDate("mmm dd, yyyy",new Date());

event.source.source.info.DocumentState = cDate

}

This code will not format the date in the style shown nor will it update the date on the stamp to the user input.  Any help?  Is there code for a date picker?