minor change to date validation

[jzaefferer]

Since datejs and others make tweaks to the Date.parse, and it's
specifically easier to override in terms of prototypal enhancement in
JS, I'd suggest updating the date validation to the following...

   // http://docs.jquery.com/Plugins/Validation/Methods/date
   date: function(value, element) {
           return this.optional(element) || !/Invalid|NaN/.test(Date.parse(value) || new Date(value));
   },

This should support situations where Date and/or Date.parse are enhanced
to handle additional parsing. :)

[fbuchinger]

I am also annoyed by the default date validator, since it does only basic syntax checking, (e.g. a german date like "99.99.2011" validates). But do we really need to depend on date.js for enhanced checking?

Since an "invalid" date object like (new Date (2008,99,99)) is converted to a future/past date (in this case to 8th march 2016), you can use the following algorithm:

1. create new Date object enteredDate from the entered year, month, day values
2. check if enteredDate.getFullYear() == enteredYear && !!enteredDate.getFullYear() (prevent falsy values,)
3. check if enteredDate.getMonth() == enteredMonth-- && !!enteredDate.getMonth()
4. same goes for Day (Date)

this could happen in a simple external function that is invoked by all culture-specific date validators once they extracted
the year, month, day values from the entered date.

[jzaefferer]

Superseded by #581