Difference in Months between two dates in JavaScript

Question

How would I work out the difference for two Date() objects in JavaScript, while only return the number of months in the difference?

Any help would be great :)

Answer 1

The definition of "the number of months in the difference" is subject to a lot of interpretation. :-)

You can get the year, month, and day of month from a JavaScript date object. Depending on what information you're looking for, you can use those to figure out how many months are between two points in time.

For instance, off-the-cuff:

function monthDiff(d1, d2) {     var months;     months = (d2.getFullYear() - d1.getFullYear()) * 12;     months -= d1.getMonth();     months += d2.getMonth();     return months <= 0 ? 0 : months; } 

function monthDiff(d1, d2) {      var months;      months = (d2.getFullYear() - d1.getFullYear()) * 12;      months -= d1.getMonth();      months += d2.getMonth();      return months <= 0 ? 0 : months;  }    function test(d1, d2) {      var diff = monthDiff(d1, d2);      console.log(          d1.toISOString().substring(0, 10),          "to",          d2.toISOString().substring(0, 10),          ":",          diff      );  }    test(      new Date(2008, 10, 4), // November 4th, 2008      new Date(2010, 2, 12)  // March 12th, 2010  );  // Result: 16    test(      new Date(2010, 0, 1),  // January 1st, 2010      new Date(2010, 2, 12)  // March 12th, 2010  );  // Result: 2    test(      new Date(2010, 1, 1),  // February 1st, 2010      new Date(2010, 2, 12)  // March 12th, 2010  );  // Result: 1

(Note that month values in JavaScript start with 0 = January.)

Including fractional months in the above is much more complicated, because three days in a typical February is a larger fraction of that month (~10.714%) than three days in August (~9.677%), and of course even February is a moving target depending on whether it's a leap year.

There are also some date and time libraries available for JavaScript that probably make this sort of thing easier.

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Note: There used to be a + 1 in the above, here:

months = (d2.getFullYear() - d1.getFullYear()) * 12; months -= d1.getMonth() + 1; // −−−−−−−−−−−−−−−−−−−−^^^^ months += d2.getMonth(); 

That's because originally I said:

...this finds out how many full months lie between two dates, not counting partial months (e.g., excluding the month each date is in).

I've removed it for two reasons:

1. Not counting partial months turns out not to be what many (most?) people coming to the answer want, so I thought I should separate them out.

2. It didn't always work even by that definition. :-D (Sorry.)