The AD calendar began as a monk's Easter table and has no year zero
In 525 a monk in Rome named Dionysius Exiguus was drawing up tables to predict Easter. Rather than keep counting years from the reign of Diocletian, an emperor who persecuted Christians, he numbered them from the incarnation of Jesus. Most modern scholars think he missed the actual birth by several years.
The old tables ran on the Diocletian era, and Dionysius simply refused to honour that tyrant's memory. His new table began with anno Domini 532, directly after Diocletian year 247. How he chose his starting point is unknown. One idea is that he worked from Luke's statement that Jesus was about thirty in the fifteenth year of Tiberius. A rival theory has him counting back 532 years, the cycle after which Easter dates repeat, a figure from the astronomer Victorius of Aquitaine. Today most scholars place the birth between 6 and 4 BC.
Adoption was slow. The system spread in England through Bede, who used it in his Ecclesiastical History of the English People, finished in 731, and on the Continent through the scholar Alcuin, whose patron Charlemagne pushed it across his empire. Popes kept dating documents by regnal years for a while, and AD only became normal in Catholic countries between the 11th and 14th centuries. Counting backwards took even longer to catch on; the Jesuit Denis Pétau popularised the Latin for before Christ in 1627.
Even agreeing on the era did not settle when a year began. Some places started it at Christmas, others on 25 March for the Annunciation, and a French royal custom used Easter. Pisa and Florence used rival March reckonings a whole year apart, so a single day might be dated 1099, 1100 or 1101 depending on where you stood. England kept a March new year until 1752.
Because there is no year zero, 1 BC is followed directly by AD 1; astronomers get around this by calling 1 BC year 0. Ethiopia, meanwhile, still counts from a different incarnation date worked out by the monk Annianus around AD 400, which is why its calendar runs seven or eight years behind.
Source: Anno Domini
