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Moment.js months difference

I’ve been using moment.js for a short while now, and it’s made date manipulation a lot easier but I have a specific case that is failing, and I can’t see why.

When calculating the diff between today (31st October 2013) and the 1st February 2014, the months diff comes back as 2, although there are 3 complete months and one day between the two dates.

Diff between 31st October and 31st January works fine: 3 months and zero days.

var mStartDate = moment([ periodStartDate.getFullYear(), periodStartDate.getMonth(), periodStartDate.getDate() ]);
var mTermDate = moment([ someDate.getFullYear(), someDate.getMonth(), someDate.getDate() ]);

console.log('periodStartDate: ' + periodStartDate);
console.log('someDate: ' + someDate);

// Years
var yearsDiff = mTermDate.diff(mStartDate, 'years');

// Months
var monthsDiff = mTermDate.diff(mStartDate, 'months', true);

The console logs the following:

periodStartDate: Thu Oct 31 2013 11:13:51 GMT+0000 (GMT)
someDate: Sat Feb 01 2014 11:13:51 GMT+0000 (GMT)
monthsDiff: 2

If I pass true as the boolean not to round, the months diff is

monthsDiff: 2.983050847457627 

Is this just a bug in Moment.js.diff()? Every single one of my other test cases pass successfully.

Answer

I think this has to do with the ‘special handling’ as described in The Fine Manual:

It is optimized to ensure that two months with the same date are always a whole number apart.

So Jan 15 to Feb 15 should be exactly 1 month.

Feb 28 to Mar 28 should be exactly 1 month.

Feb 28 2011 to Feb 28 2012 should be exactly 1 year.

Moment.js applies this special handling when dealing with 31 Jan and 31 Oct (having the same day):

// 31 Oct 2013 - 1 Feb 2014
> moment([2014, 1, 1]).diff(moment([2013, 9, 31]), 'months', true)
2.983050847457627

// 31 Oct 2013 - 31 Jan 2014
> moment([2014, 0, 31]).diff(moment([2013, 9, 31]), 'months', true)
3

// 31 Oct 2013 - 30 Jan 2014
> moment([2014, 0, 30]).diff(moment([2013, 9, 31]), 'months', true)
2.967741935483871

So the 2.98 value is correct, it’s just that the second example turns the result into a ‘calender months’ difference.

(as for rounding down to 2, that’s also documented on the same page)