Why, if everything else in JS is 0 indexed? Isn't consistency a good thing in a language? If they'd gone the other way, wouldn't there be people here complaining that 1 indexing is whack?
Well because days are returned 1-indexed and calendars in general are 1-indexed, but of course the internal 'getMonth' method isn't meant to be a calendar, it's meant to provide an array index to ["January", "February", ...] as an interface to the other methods of printing a human-readable form.