[etothepii]

This will never stop as it would require either the reference date to be changed or fir all dates in all saved spreadsheets to be off by one.

[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...

[zvr]

IIRC, it will never stop because now it has been "standardized". The OOXML specification explicitly mentions that the conversions of dates and their attributes (like whether a year is a leap one) has to follow the Excel implementation.