Add Time2.1.0+

moment.duration().add(Number, String);
moment.duration().add(Number);
moment.duration().add(Duration);
moment.duration().add(Object);

Mutates the original duration by adding time.

The same keys and shorthands used to create durations can be used here as the second argument.

var a = moment.duration(1, 'd');
var b = moment.duration(2, 'd');
a.add(b).days(); // 3

Note that adding an invalid duration to any other duration results in an invalidduration.