Are you on mobile? The question of timekeeping comes up a lot in history, so the FAQ has quite a few answers for you.

https://www.reddit.com/r/AskHistorians/wiki/calendars

For example: https://www.reddit.com/r/AskHistorians/comments/wu47z/who_or_what_group_of_people_decided_the_switch/ by /u/Algernon_Asimov

Not to discourage further discussion of calendars. It might be nice to have an answer the brings together all the threads in the FAQ.