When did people start counting with the birth of Christ as the starting date?

Answer

The Christian era as we know it today can be found for the first time in an Easter table from 525 AD. In that Easter table was kept for each year when Easter was celebrated. The maker of this Easter table, Dionysius Exiguus, based himself on an older Easter table, but he replaced the first column (with the regnal years from Diocletian, a notorious Christian persecutor) by a column with an era from the birth of Christ. The first year of his Easter table was then indicated by 532, the second year by 533, etc…

How he calculated that the 285th year of Diocletian’s reign was equal to the 532nd year from Christ’s birth, we do not know with certainty. You can read interesting theories about this in Georges Declercq, Anno Domini. The Origins of the Christian Era.

This Easter table was drawn up in Rome and has slowly found its way into the Latin West. Thanks to the accuracy of the chronological data (including the date for Easter Sunday), the corresponding era became more and more known. It was not until about the year 1000 that the Christian era was accepted as the usual method of dating in the historiography of that time.

The Christian era of Dionysius Exiguus is not correct. We know with some degree of certainty that Herod the child killer died in 4BC. In other words, Jesus was born a few years before the beginning of our era. When exactly, we don’t know…