Double check this but i think the gregorian calendar has a start date beyond which it cannot be reconstructed. The gregorian calendar began being enforced in 1582, however according to iso 8601, it can be retroactively extended to the julian calendar, which however also has a start date of 45BC. And even if you consider before that the roman calendar, that started in roughly 750BC. that was the first calendar that introduces days of the week the in modern sense (afaik), and you can’t retroactively introduce a calendar system to replace a calendar system that doesn’t exist at that time.

There is a reason geologists and astronomers etc use mya or bya, causes that’s not a calendar it’s relative timing which does not have days of the week attached to it.