// Adding date difference functionality to default date Object
Date.prototype.difference = function(secondDate, units) {
  //   Start date and end date should be actual Date objects
  var dateDifference = this - secondDate;
  switch (units) {
    case "seconds":
      dateDifference = dateDifference / 1000;
      break;
    case "minutes":
      dateDifference = (dateDifference / 1000)/60;
      break;
    case "hours":
      dateDifference = ((dateDifference / 1000)/60)/60;
      break;
    case "days":
      dateDifference = (((dateDifference / 1000)/60)/60)/24;
      break;
    case "months":
      dateDifference = this.getMonth() - secondDate.getMonth();
      break;
  }
  return dateDifference +" " +units;
}

var date1 = new Date('7/25/2010');
var date2 = new Date('12/12/2010');
// Since date is a static object you will always need to access it using .prototype in the call
var interval = date2.difference(date1, "days");
console.log(interval);