[xelxebar]

I believe you fail to grok GP's point.

The mapping from "month" units to "days" units is well-defined. It's just some function that happens to be non-constant over the month ordinal.

Heck, if we start down the "precise time definition" rabbit hole, though, then neither does "day" have some philosophically unassailable notion, cf. leap seconds. Even the unit of "second" ends up pulling in a whole heck of a lot of physics machinery just to nail down some semblance of rigor.

Anyway, despite being such an intuitively simple and practically functional concept, the notion of time amd time measurement turns out to be suprisingly subtle and to have a fascinating history. I highly recommend jumping down that rabbit hole. Hehe

Anyway, I'm surprised calc.exe doesn't calculate with and store dates using some kind of epoch time.