I’m pretty sure that when “Jesus was born” that there was not a global announcement that it was know 0, so how did people know the year. Also, people in like 150ad, how would they know?
Years have been counted in different ways over the world through history, though very often by who was in power in a society’s highest position: this is called the regnal year. Here is a pretty clear overview of how that works.
So in the Roman Empire at the time of Jesus’s birth, for instance, years were commonly identified by the year of an emperor’s reign: “Year X of Emperor So-and-so.”
The Japanese still use this traditional method of dating years, alongside what has become the standardized Common Era system: in Japan you could say this year was 2022, or you could say it was Reiwa 4 — the fourth year of the Reiwa Emperor, who ascended in 2019.