Jesus Birth Date Year

How did historians date time before Christ? Did they have to go back and affix dates after Jesus’ birth?

I know there was recorded history many years before Christ. How were those records dated? Did the people who wrote them put a date on them or was it just according to who was reigning at the time?

In Christ’s time the Romans numbered their years anno urbis conditae, from the founding of the city [of Rome]. Christ was born circa 750 AUC. Other systems of reckoning were also used from time to time.

Was there a massacre around the time of Jesus birth?

The Julian, Gregorian, and Jewish calendars (55 B.C. to A.D. 2169): A calculation of the birth year of Jesus : a chronology of the passion of Jesus …