[organsnyder]

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.