How did humans measure years before Jesus? At what point did they believe was the beginning of time? Or did they keep track of years at all?
There's always room for discussion, but perhaps the section Calendars and timekeeping in our FAQ will answer your inquiry.
One thing to note is that the use of AD/BC did not arise until medieval times.
In the Roman Empire, in which early Christian history mainly takes place, the dating system was that of Rome, which counted from the founding of the city in 753 BCE. The year 1 AD was 754 AUC (ab urbe condita / from the founding of Rome).