How to correctly use JavaScript indexOf in a date array

Question

Here is the code:

var collection = [new Date(2014, 11, 25), new Date(2014, 11, 24)];
var d=new Date(2014, 11, 24);

var idx= collection.indexOf(d);

Answer 1

Array.prototype.indexOfDate = function(date){
   for (var i = 0; i < this.length; i++){
      if (+this[i] === +date) return i;
   };
   return -1;
};

// then 
var idx1 = collection.indexOfDate(d);