You're right, but I think the problem does arise for "four and a half months away". That actually requires you to pin down how many days are in a month.
There is -- or should be -- an understanding that when such a phrase is used, it is not intended to be taken as a precise duration. Furthermore, for any of these expressions, while there is not necessarily just one moment in time that it is equivalent to, there is a vast range of times that it definitely does not denote.
It only requires you to know how many days are in each month, and you have to know what you’re counting. That’s the problem with calc: it discards the semantics of “how I got here,”
I think it's more complicated, and depends on assumptions made by the speaker and the listener: if I say "two and a half months from now", is the half-month equal to 15 days or 14? If the target duration is February, perhaps I'd assume 14; but otherwise I probably assume 15; but what if February is one of the whole months in between?
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.
True, but you wouldn't use that in a context where precision is required. You'd say something like '31 weeks' if the specific duration mattered, or just the date itself. When the exact duration isn't particularly relevant, the ambiguous 'four and a half months' works just fine.