The switch to the current system of using the theoretical birth of Jesus as the start of our calendar occurred in the 6th century, 500 years after the fact. They picked a year based on what evidence they had for when the birth of Jesus occurred with a margin of error of about ~30 years.
When this occurred and we started observing years in Anno Domini, whatever local calendar was being used was immediately replaced by the year 525, and retroactively everything before that was assigned it’s proper year. This ends with AD 1 and directly starts with BC 1 going the other direction. No year 0 was observed in this switch.
Also note that before this switch, years were often designated in relation to the founding of a city or by the start of a ruler’s reign. There were always ordinal numbers, so the first year of a reign would be year 1, and there was never a zero, because it was year X of a previous reign.
The switch to the current system of using the theoretical birth of Jesus as the start of our calendar occurred in the 6th century, 500 years after the fact. They picked a year based on what evidence they had for when the birth of Jesus occurred with a margin of error of about ~30 years.
When this occurred and we started observing years in Anno Domini, whatever local calendar was being used was immediately replaced by the year 525, and retroactively everything before that was assigned it’s proper year. This ends with AD 1 and directly starts with BC 1 going the other direction. No year 0 was observed in this switch.
Also note that before this switch, years were often designated in relation to the founding of a city or by the start of a ruler’s reign. There were always ordinal numbers, so the first year of a reign would be year 1, and there was never a zero, because it was year X of a previous reign.