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EDIT: This is not a duplicate of round up/ round down a momentjs moment to nearest minute - because first I don't want to round to nearest minute; second, I don't want to round unconditionally, but only when the difference to .endOf('day') is not the whole hours I'd otherwise expect; and third, I want to round a moment.duration, not a moment.

Say I have a date/time stamp, "2017-02-17 21:00:00" and I want to find how many hours there are to the end of day. Mentally, if I think of 9 o'clock in the evening, I consider it 3 hours away from midnight, and that is the answer I'd want to obtain from momentjs. This is what I am doing (Javascript Web Console in Firefox):

var m1 = moment('2017-02-17 21:00:00');

<- undefined



var m2 = moment(m1).endOf('day');

<- undefined



m1

<- Object { _isAMomentObject: true, _i: "2017-02-17 21:00:00", _f: "YYYY-MM-DD HH:mm:ss", _isUTC: false, _pf: Object, _locale: Object, _a: Array[7], _d: Date 2017-02-17T20:00:00.000Z, _isValid: true, _z: null }



m2

<- Object { _isAMomentObject: true, _i: "2017-02-17 21:00:00", _f: "YYYY-MM-DD HH:mm:ss", _isUTC: false, _pf: Object, _locale: Object, _z: null, _a: Array[7], _d: Date 2017-02-17T22:59:59.999Z, _isValid: true }



var mdiff = moment(m2).diff(moment(m1))

<- undefined



mdiff

<- 10799999



var mddur = moment.duration(moment(m2).diff(moment(m1)))

<- undefined



mddur

<- Object { _isValid: true, _milliseconds: 10799999, _days: 0, _months: 0, _data: Object, _locale: Object }

So far, so good - now, to format the duration, I go as per Get the time difference between two datetimes (also duration formatting · Issue #1048 · moment/moment · GitHub); note that I want to use the same function I'd use to get correct durations larger than 24 hours to calculate this - even if this particular example has a duration shorter than 24h, so I use this:

Math.floor(mddur.asHours()) + moment.utc(mddur.asMilliseconds()).format(":mm:ss")

<- "2:59:59"

So, here I's want to obtain the answer "3:00:00" here, not "2:59:59" - though note, I'd still want "2:59:58" as is, and not rounded up.

I guess, if our duration in ms is 10799999, that is 10799999/1000=10799.999000 seconds, so if we have a duration that has millisecond remainder of 999 milliseconds, only then I would want a round up.

What would be the recommended way of achieving this with moment.js?

Ok, I ended up making a function that does what I want, here it is:

function padDigits(number, digits) { // SO:10073699

  return Array(Math.max(digits - String(number).length + 1, 0)).join(0) + number;

}



var durationToHoursStr = function(induration) {

  // copy the duration

  thisduration = moment.duration(induration);

  // to get seconds, we divide milliseconds with 1000;

  // check if the remainder of division of milliseconds with 1000 is 999:

  var remainder = thisduration.asMilliseconds() % 1000;

  //console.log("remainder", remainder);

  // only round up if the remainder is 999:

  if (remainder >= 999) {

    // in call moment.duration(Number), Number is interpreted as "length of time in milliseconds" (https://momentjs.com/docs/#/durations/)

    // so just add 1 ms, since anyways the previous remainder is 999, should be enough to flow over

    thisduration.add(moment.duration(1)); // should be in-place replacement - mutable

    //console.log("thisduration", thisduration);

  }

  return String(Math.floor(thisduration.asHours())) + ":" + padDigits(thisduration.minutes(), 2) + ":" + padDigits(thisduration.seconds(), 2);

};



var m1 = moment('2017-02-17 21:00:00');

var m2 = moment(m1).endOf('day');



// mdiff is int, total number of milliseconds, here 10799999

var mdiff = moment(m2).diff(moment(m1))



// mddur is moment.duration, also has the same amount of milliseconds, 

// mddur.asMilliseconds() = 10799999, mddur.asSeconds() = 10799.999, mddur.asHours() = 2.9999997222222223

var mddur = moment.duration(moment(m2).diff(moment(m1)));



var mddur_hoursString = durationToHoursStr(mddur);

console.log(mddur_hoursString); // prints "3:00:00"



var m3 = moment('2017-02-17 23:59:59');

var mddur3 = moment.duration(moment(m3).diff(moment(m1)));

var mddur3_hoursString = durationToHoursStr(mddur3);

console.log(mddur3_hoursString); // prints "2:59:59" - as it should, since we didn't really demand the difference to the end of day