Adding Days to Dates in Javascript

There's no built-in function in the Javascript Date object that adds days to an existing date. However, there's a way to make it work by using the Date contructor.

    var date = Date(year, month, day);

Using this constructor, we can acomplish this task. Let's say that we're having a date variable called today and you want to alert a date that is 15 days after this date. The following should do the job.

function displayDates() {
        var today = new Date();
        var date = Date(today.getFullYear(), today.getMonth(), today.getDate() + 15);
        // Remember that today.getDate()
        function retrieves the days part of the date

Don't worry, if today is October, 25 2010, Javascript will automatically realize that adding 15 days to this date will move us to the next month; November.

In order to make our task a little bit simpler amd more reusable, let's make benifit of the Date's prototype and add a function to the Date objects. Add the following to the same page.

Date.prototype.AddDays =  function(numberOfDays) {
        return new Date(this.getFullYear(), this.getMonth(), this.getDate() + numberOfDays);

This will allow us to easily add days to any date that we make in the future. The following code illustrates how to use it.

function displayDates() {
        var today = new Date();

To complete the set of functions, add the following as well.

Date.prototype.AddMonths =  function(numberOfMonths) {
        return new Date(this.getFullYear(), this.getMonth() + numberOfMonths, this.getDate());
    Date.prototype.AddYears =  function(numberOfYears) {
        return new Date(this.getFullYear() + numberOfYears, this.getMonth(), this.getDate());

Of course, you know that if what we need to do is subtracting days, then we can send a negative numberOfDays value.

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