[gpm]

Or... A version number that tells excel which convention to use?

I suppose that would make copy and pasting formulas between spreadsheets very mildly error prone though, so it probably won't happen.

[gus_massa]

There already two date systems, IIRC for compatibility with Macs, I probably read it in the OldNewThing, but I can't finf it. Anyway, it's explained in https://support.microsoft.com/en-us/office/date-systems-in-e...