they do when you use signed 64bit timestamps (or 128bit ones if you need the nanos)

Apparently it's technically possible in Git as of a year or two ago, with some conversion bugs probably still lurking, but Github/Gitlab still don't support it. And the frontend tools like "git commit" don't support it. The project in this answer is using git hash-object to create the commit from raw bytes. https://stackoverflow.com/questions/21787872/is-it-possible-...

Unfortunately git rejects dates before 1970, at least when I set them using GIT_AUTHOR_DATE

This is especially troublesome when you need to cover up those 50 years of procrastination on a project.

You say 'procrastination' I say 'The means IS the end, yo!'.