Fix for issue 261.

[damienwhaley]

It appears when you are working with two moment objects and you take the diff, the input moment's _isUTC is ignored when you parse the input into inputMoment. I've done a check to see if it looks like a moment object (by checking the _isUTC to see if it is defined) and if it is, then it just copies the input rather than parsing it. I think it should work fine on regular date objects as there is no _isUTC member.

[rockymeza]

I think you might mean input._isUTC instead of moment._isUTC. The moment global does not have an _isUTC flag on it, only instances have the flag. There is a function available if you want to typecheck moment instances, moment.isMoment. However, you said that a moment's _isUTC flag is ignored when you call the moment function on it. Maybe the true bug lies in the moment function.

[timrwood]

Yeah, I agree, if moment(moment.utc()) isn't copying the _isUTC flag, then that is where it should be fixed.

https://github.com/timrwood/moment/blob/71eda19de4a277b76419d18f233ac4877bf801f2/moment.js#L416 should change to something like this:

var isUTC;
...
if (moment.isMoment(input)) {
    date = new Date(+input._d);
    isUTC = input._isUTC;
}
...
return new Moment(date, isUTC);