In ancient times, dates were usually the Xth year of some king's reign. The Holy Roman Empire declared Jesus as their king and made the calendar accordingly, which spread throughout the west.
Various revolutionaries naturally want to put themselves on the top or are otherwise hate Christian culture and therefore have tried to change the calendars or change what the BC/AD/BCE/CE acronyms stand for or whatnot due to their desire to exclude Christianity from their cultures.
There are plenty of other calendars in existence, though. Japan still has eras based on their emperor at the time, for example, though the western calendar is a lot simpler because you don't have to memorize a huge list of emperors and do a lot of math to figure out how many years ago something happened. Quick, how long ago was Showa 55 if Showa went up to 64, Heisei up to 30, and 2020 is Reiwa 2?
> The Holy Roman Empire declared Jesus as their king
In west tradition it is often INRI inscribed above Jesus head. That stands for "Iesvs Nazarenvs Rex Ivdæorvm". Not king of Romans, but king of Jews. That was declared as His fault, because that time Judea was a province of the Roman Empire, not independent kingdom, so when one declare himself as a king, he really declare rebellion. This was used by the Jews to force Pilate to execute Him.
Implicit side of this: He was really the King of Jews and He put the end of Old Testament history.
Birthday of Jesus Christ was accepted as a starting point for calender several hundred years after His death and Resurrection. Roman Empire does already transformed to Byzantine Empire at that time.
Nevertheless in Byzantine Empire commonly was used calendar since creation of the universe. And in Russia also, till 1700 AD.
So it is not such a straightforward acception of the calender, but status of calender nowadays as a status of English language, most of us understand it, even if not use in everyday life.
And it is really important, because makes the birthday of Jesus Christ just one day in history, which in turn makes Him real person, non fictional, non mythological.
Various revolutionaries naturally want to put themselves on the top or are otherwise hate Christian culture and therefore have tried to change the calendars or change what the BC/AD/BCE/CE acronyms stand for or whatnot due to their desire to exclude Christianity from their cultures.
There are plenty of other calendars in existence, though. Japan still has eras based on their emperor at the time, for example, though the western calendar is a lot simpler because you don't have to memorize a huge list of emperors and do a lot of math to figure out how many years ago something happened. Quick, how long ago was Showa 55 if Showa went up to 64, Heisei up to 30, and 2020 is Reiwa 2?
And that's an easy one.
https://www.ewc.co.jp/Pages/Information/CalendarEN.aspx
Yeah, it was 1980, so that was 40 years ago. How many got the overlapping wrong and were off by 1, though?