JavaScript DateDiff Extension Methods

When working with Date types in JavaScript, we often have the need to do date difference calculations. An approach to provide this functionality would be to extend the Date type with functions that can handle the calculations. So let’s talk about how to create these extension methods.

We can create extension methods on any JavaScript object by using the Object.prototype property. This property allows us to associate dynamic properties and functions to an object. This is what we use to associate the following functions to any Date instance.

Extension methods:

Date type Extension Methods
Source: ozkary.blogspot.com
Date: 4/27/2014
//Month difference
Date.prototype.monthDiff = function (dtBefore) {
      var yr = (this.getFullYear() - dt.getFullYear()) * 12;
      var mn = ((this.getMonth() + 1) - (dt.getMonth() + 1));
      return yr + mn;

//year difference
Date.prototype.yearDiff = function (dtBefore) {
      var yr = (this.getFullYear() - dt.getFullYear());
      return yr;

//date format
Date.prototype.toShortDate = function () {
      var dt = (this.getMonth() + 1) + '/' + this.getDate() + '/' + this.getFullYear();
      return dt;

Function name
monthDiff (date)
This extension method expects a date type as a parameter. It uses native functions to calculate the years and months difference between itself (this) and the date parameter.
yearDiff (date)
This extension method expects a date type as a parameter. It uses native functions to calculate the year difference between itself (this) and the date parameter
This extension method uses the native functions to extract the date parts (month, day, year) and return the short date format (mm/dd/yyyy)


Now that we have our new functions available, we can work on showing a way to use them. We can do that by comparing a couple of date ranges to display the difference in months and years. In our examples, we use the most recent date as the extended object, and pass the before date as the parameter. This allows us to get positive values for the date difference calculation.

var date1 = new Date('2/16/2014');
var date2 = new Date('09/01/2009');
$('#dates1').text(date1.toShortDate() + ' - ' + date2.toShortDate());
//get the date difference
var m = date1.monthDiff(date2);
var y = date1.yearDiff(date2);

//get the date difference
date1 = new Date('3/28/2014');
date2 = new Date('12/28/1996');
$('#dates2').text(date1.toShortDate() + ' - ' + date2.toShortDate());
m = date1.monthDiff(date2);
y = date1.yearDiff(date2);

This script performs the following steps:
  • Create Date instances
  • Display the short date format (mm/dd/yyyy)
  • Calculate the months and year differences

The result should look as shows below. We can also use this JSFiddle sample to see it in action.

Thanks for following and keep an eye for additional entries.