AddMonths bug

[Nadoedalo]

You can't chain-call the addMonths, i.e new Date().addMonths(1).addDays(-1) because addMonths returns undefined.
Those functions should return the result(i.e , not only modify the actual date but also return that date object).

Maybe return this; will help

[nazar-pc]

Why should they?
They behave just like native methods of Date object, native methods doesn't support chaining as well.