The Python "dateutil" library includes a "relativedelta" that works how one would typically expect. Though, I've usually used it to add known deltas, not to subtract two dates and arrive at a delta, so IDK if it supports that.

(It has rules, of course, for dealing with 30 day months when you're on the 31st and you add a "month". The normal timedelta class from the standard library doesn't do "months", since, as you note, it's not really a measure of time.)

In Unix timestamps, over a leap second, a difference of 1 Unix second is occasionally 2 seconds long.