Further questions:

Time is generally measured by counting up from an event, although I'd guess some astronomical system of measurement may have counted down, but from what? What was the starting point for different civilizations concept of "year one," and why was that so significant to them?

When did the "years since jesus" system become widely accepted? How long did it take to spread everywhere? I would expect that, particularly in Asia, they wouldn't have bothered switching to a year system based on a guy they didn't care about until fairly recently, probably when the British rolled up. Case in point, Chinese new year.