The javascript Date object and how to add days to a date variable

JavaScript is not an object oriented language, it’s a prototype based language. This means that you can extend any existing object by just writing a new function for it on-the-fly. Here’s an example:

// add n number of days
Date.prototype.goto = function(n) {
	return this;

Note: Thanks to commenter Joe I use setDate instead of setTime.
Yes I know… GOTO is considered harmful. But so is Edsger Dijkstra. Anyway, this nifty little function will add, or subtract(!), any number of days with an easy to use syntax:

// add n number of days
d = new Date; // today
d.goto(7); // move it to next week
d.goto(-365); // move to last year (well if it isn't leap)

Try it yourself, it’s fun. Now to make things more interesting, you can make a generic function (erm.. method?!) that will calculate the number of days since the Epoch

Date.prototype.days = function() { return parseInt(this.getTime() / (1000*24*60*60)); };

This makes stuff like comparing dates and calculating differences way easier. E.g. check out this one for calculating the number of working days between two dates:

Date.prototype.workdays = function(d2) {
	var start = new Date(this);
	var end = d2;
	var ret = 0;
	var diff = end.days() - start.days();
	if (diff > 0)
		// up to 1 year
		while (start.days() < end.days() && ret < 3650)
			start.goto(1); // move to next day
			if (start.getDay() != 0 && start.getDay() != 6)
	return ret;

5 comments so far

  1. Álvaro G. Vicario on

    The problem with the “add 86400 seconds” approach is that days have 24 hours… or 23… or 25… thanks to Daylight Saving Time.

  2. Sam on

    Yup, fails due to daylight savings time:

    >>> (new Date(2010, 9, 8)).goto(30);
    Sun Nov 07 2010 00:00:00 GMT-0400 (Eastern Daylight Time) {}
    >>> (new Date(2010, 9, 8)).goto(31);
    Sun Nov 07 2010 23:00:00 GMT-0500 (Eastern Standard Time) {}

  3. Paul Russell on

    Thanks for that code. This works and fixes the Daylight Saving issue.

    Date.prototype.goto = function(n) {
    var thisUTC = this.getTimezoneOffset();
    if (thisUTC!=this.getTimezoneOffset()) { this.setTime(this.getTime()+(this.getTimezoneOffset()-thisUTC)*60000); }
    return this;

    • Joe on

      What’s wrong with just using setDate() instead of setTime? Adding to the date (day of the month) will automatically adjust the month & year.

      So if it’s 28th Sept and you do x.setDate(x.getDate()+3), the date in x will be 1st Oct.

      • michiel on

        Yes I think I saw this before… I was assuming setTime would do the same. Since it doesn’t the code is not to be trusted. I’ll rewrite based on your input. Thanks, Joe.

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