Get the time difference between two datetimes

Your problem is in passing the result of moment.duration() back into moment() before formatting it; this results in moment() interpreting it as a time relative to the Unix epoch.

It doesn't give you exactly the format you're looking for, but

moment.duration(now.diff(then)).humanize()

would give you a useful format like "40 minutes". If you're really keen on that specific formatting, you'll have to build a new string yourself. A cheap way would be

[diff.asHours(), diff.minutes(), diff.seconds()].join(':')

where var diff = moment.duration(now.diff(then)). This doesn't give you the zero-padding on single digit values. For that, you might want to consider something like underscore.string - although it seems like a long way to go just for a few extra zeroes. :)

This approach will work ONLY when the total duration is less than 24 hours:

var now  = "04/09/2013 15:00:00";
var then = "04/09/2013 14:20:30";

moment.utc(moment(now,"DD/MM/YYYY HH:mm:ss").diff(moment(then,"DD/MM/YYYY HH:mm:ss"))).format("HH:mm:ss")

// outputs: "00:39:30"

If you have 24 hours or more, the hours will reset to zero with the above approach, so it is not ideal.

If you want to get a valid response for durations of 24 hours or greater, then you'll have to do something like this instead:

var now  = "04/09/2013 15:00:00";
var then = "02/09/2013 14:20:30";

var ms = moment(now,"DD/MM/YYYY HH:mm:ss").diff(moment(then,"DD/MM/YYYY HH:mm:ss"));
var d = moment.duration(ms);
var s = Math.floor(d.asHours()) + moment.utc(ms).format(":mm:ss");

// outputs: "48:39:30"

Note that I'm using the utc time as a shortcut. You could pull out d.minutes() and d.seconds() separately, but you would also have to zeropad them.

This is necessary because the ability to format a duration objection is not currently in moment.js. It has been requested here. However, there is a third-party plugin called moment-duration-format that is specifically for this purpose:

var now  = "04/09/2013 15:00:00";
var then = "02/09/2013 14:20:30";

var ms = moment(now,"DD/MM/YYYY HH:mm:ss").diff(moment(then,"DD/MM/YYYY HH:mm:ss"));
var d = moment.duration(ms);
var s = d.format("hh:mm:ss");

// outputs: "48:39:30"

var a = moment([2007, 0, 29]);
var b = moment([2007, 0, 28]);
a.diff(b, 'days') //[days, years, months, seconds, ...]
//Result 1 

Worked for me

See more in http://momentjs.com/docs/#/displaying/difference/