There are a ton of different year calendars. The BC/AD calendar is very much a Western/Western-derived thing, stemming from a Catholic Christian estimation. It’s convenient, because it got widespread, but it’s only one system of many.

Many ancient systems judged the date by the regnal year of the ruler, ie “Third year of the rule of Agum the Third”. It’s much less convenient for figuring out when something happened than systems that continuously count from a single starting year, but exact chronologies of events in the distant past were less important for most people.

https://en.wikipedia.org/wiki/Calendar_era

Sounds like a question for @[email protected]