Note: This question comes in a few parts.

We're just over a month away from 2019, two thousand and nineteen years since the birth of Christ.

As historians events are measured by BC and AD with BC counting down to the birth of Christ and everything after counting upwards.

1. Obviously before the birth of Christ civilisations weren't counting down to something as they would not know what they are counting down to so how did they deal with years on a calendar? Or did they not? Say, in Rome BC, what would they put as the date when writing a document and what event were the years in their calendars based off of?
2. How long after the birth of Christ did someone decide that that should be what we count our years from and how can we judge if this is accurate?
3. At what point after the birth of Christ did all Christian nations agree to change their calendar system?
4. How long did it take before the Gregorian calendar became the most widely used calendar?

Thank you

Note: the question of whether or not Jesus was real or not isn't the debate. I'm just interested in our calendar.