The same thing it means when something is 2 years away when the years could be different lengths?
The month is a well-defined unit within the context of a certain calendar. And since a normal date representation contains the month as a singular component, month additions are usually well-defined (the obvious exception being, adding N months to yyyy-mm-31 -- the result is undefined, though most people will assume it to refer to the first day of the month following).
There is no need to bring mathematical strictness into it, because months are never used for that purpose. Moreover, no calendar system is built for mathematical strictness anyway, so arguing for that kind of strictness is like building a house on quicksand.
I wouldn't, I'd assume it's the last day. For me, any date that is "two months after" any date in July should fall in September, with the days longer than the target month scaling down. So two months after July 31 should be, to me, September 30.
But,
$ date -d "2019-07-31 +2 months"
Tue, Oct 1, 2019 12:00:00 AM
> (date 2019-07-31).AddMonths(2)
Montag, 30. September 2019 00:00:00
For software libraries it's a bit of a pickle, though, as you have the trade-off between »do what an end user would expect« and »do exactly as told, and let the programmer deal with the weirdness«. Admittedly, adding months is something that I'd rarely do as a programmer when the context isn't what a user would expect. Adding 30 days can be done in much the same way if that's really what's intended.
Interesting question, though: Is 2 months from 2019-07-28 2019-09-28 because the day part is the same, or 2019-09-27 because it's three days from the end of the month? ;-)
In my experience (in my culture), we avoid these problems by just assuming that we'd never convey a precise date in this manner: anything more than a week away we tend to convey with the exact date (March 15th); or, we might say "exactly two weeks from today" or "exactly one month ago" (implying the same numerical day-of-month, regardless of how long each month is), but I don't think we'd ever say "exactly one and a half months from now".
And this is why calendars are hard: it's as much (or more) anthropology as it is arithmetic.