add & subtract method inconsistency on negative and positive floating numbers with bigger units

[huttli]

m().add(1.2, 'months').toString();
'Sun Dec 20 2015 12:12:57 GMT+1030'
m().add(1, 'months').toString();
'Sun Dec 20 2015 12:13:01 GMT+1030'
m().add(-1, 'months').toString();
'Tue Oct 20 2015 12:13:06 GMT+1030'
m().add(-1.2, 'months').toString();
'Sun Sep 20 2015 12:13:10 GMT+0930'

I don't expect the floating numbers to be exactly calculated into smaller units fractions given the known two issues here:
#2305
#2430

But adding with negative floating number seems give the wrong month, too. I'd expect the last line to be
'Sun Oct 20 2015 12:13:10 GMT+1030'

A Math.round() vs Math.floor() difference?

I live in an area doing DST change in early Ocotober, please ignore the GMT offset differences. :)

[huttli]

The same for subtract with a positive floating point number.

We're relying on the native Date.setMonth/setUTCMonth(https://github.com/moment/moment/blob/develop/moment.js#L693) to convert the floating point number to integer number, where ECMA defines as Math.floor with its absolute value.
http://www.ecma-international.org/ecma-262/5.1/#sec-9.4

So subtract 1.2 months from November becomes converting (10-1.2) to integer, which becomes 8, hence the September result.

I still expect our feature rich momentjs to perform a bit more intelligent than the native Date object. :D

[icambron]

I think we'd take a PR for this, sure.

[mattjohnsonpint]

We'll track in PR #2970. Thanks.