I think you misunderstand me. I mean ONLY the specific idiom “the 20th century” because we’ve just spent more than a century using that and it is unambiguous, i

I think you misunderstand me. I mean ONLY the specific idiom “the 20th century” because we’ve just spent more than a century using that and it is unambiguous, it’s clear. Use “the 1700s” and the like for all other cases.