I know at least until second and third centuries the Romans would have used the founding of Rome as their start date. I presume most other civilizations had start dates like this, or a Ruler relative system e.g. "We are in 2nd year of A's rule, and before that we were ruled by B for 15 years. Thus 'year 2' means multiple years."

So when did and how did the use of "A.d." and "B.c." start to spread?