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JavaScript date difference calculation

Calculating date difference in JavaScript is almost as easy as in PHP, but requires a bit different approach. For this post i wrote simple “static” js object which can calculate difference between two dates in: days, weeks, months and years.

To each function you pass two arguments/dates, the first one should be lower then second, or you will get negative number on the output, however the number will still be valid, although with negative sign, so you just need to multiply it by “-1”.

Calculating date difference in days and weeks works pretty much the same. First we get difference between UNIX timestamps for first and second dates and then divide it by number of milliseconds in a day (for days calculation), or by number of milliseconds in a week (for weeks calculation).

Number of milliseconds in a month vary (because months have different number of days), so we are taking a different approach. For each date multiply year by 12 and add current month, that gives total number of months since year 0. Now substract both numbers and we are done.

Calculating difference between years i leave without a comment :). Source code with example usage below.

var DateDiff = {
    inDays: function(d1, d2) {
        var t2 = d2.getTime();
        var t1 = d1.getTime();
        return parseInt((t2-t1)/(24*3600*1000));
    inWeeks: function(d1, d2) {
        var t2 = d2.getTime();
        var t1 = d1.getTime();
        return parseInt((t2-t1)/(24*3600*1000*7));
    inMonths: function(d1, d2) {
        var d1Y = d1.getFullYear();
        var d2Y = d2.getFullYear();
        var d1M = d1.getMonth();
        var d2M = d2.getMonth();
        return (d2M+12*d2Y)-(d1M+12*d1Y);
    inYears: function(d1, d2) {
        return d2.getFullYear()-d1.getFullYear();
var dString = "May, 20, 1984";
var d1 = new Date(dString);
var d2 = new Date();
document.write("<br />Number of <b>days</b> since "+dString+": "+DateDiff.inDays(d1, d2));
document.write("<br />Number of <b>weeks</b> since "+dString+": "+DateDiff.inWeeks(d1, d2));
document.write("<br />Number of <b>months</b> since "+dString+": "+DateDiff.inMonths(d1, d2));
document.write("<br />Number of <b>years</b> since "+dString+": "+DateDiff.inYears(d1, d2));
Published inGeneral


  1. thanks bro.. it works..

  2. Srini Srini

    Nice simple and effective. Thanks for sharing!!

  3. Darryl Waterhouse Darryl Waterhouse


    Try March, 27, 2011 to March, 28, 2011.
    It yields zero.

    I can not find a script anywhere that gets around this issue. And every Javascript engine (on every browser) fails to yield a value of one. :(

  4. Darryl Waterhouse Darryl Waterhouse

    It appears that there is a calculatory math problem in Javascript.
    Adding an arbitrary half when returning calculated values solves this problem.

    return parseInt((t2-t1)/(24*3600*1000) + .5);

    Note the addition of the “.5”, which overcomes the issue.

    Please feel free to correct/comment.


  5. Great post, keep up the good work.

    Also thanks Darryl for the bug fix.

  6. daniel daniel

    It is 2011/05/18 Today.

    I change value of dString in source code as following
    var dString = “2011/04/30”

    This result in “Number of months since 2011/04/30: 1”

    I think the number of months is less than 1.

  7. john john

    Thanks a lot, it saved my time.

  8. Very clean. Adding hours, minutes and seconds was a breeze. Cheers.

  9. Dmitriy Dmitriy

    Hehe and what happens if you will try to calculate the difference between :

    1 September 2011 and 1January 2012

    It will give you incorrect result – 1 year but the correct result will be 0 years and 4 months.

    Use the javascript Date libraries – they do the calculations for you correctly.

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