Answer by pixelbacon for Detecting an "invalid date" Date instance in JavaScript

Generally I'd stick with whatever Date implantation is in the browser stack. Which means you will always get "Invalid Date" when calling toDateString() in Chrome, Firefox, and Safari as of this reply's date.

if(!Date.prototype.isValidDate){
  Date.prototype.isValidDate = function(){
    return this.toDateString().toLowerCase().lastIndexOf('invalid') == -1;
  };
}

I did not test this in IE though.