When Anno Domini chronology was introduced?
from the series of short articles about calendars
The supposed birth year of Jesus Christ as point of reference of the contemporary calendar (Gregorian as well as Julian Calendar) was established 6 years before the conclusion of Cyrillian Paschal (Easter) cycle on Diocletian year 247 (year after the reign of Diocletian year 1 corresponds to the year from August 29th, AD 284 until August 28th, AD 285). The man who introduced the new reference point for year numbering was the Scythian monk Dionysius Exiguus (AD 470-544) or Dionysius the Humble. Diocletian year 247 following our current chronology was ending in AD 531; consequently the year "6 years before that year" was AD 525. This was the year selected by Dionysius to establish his innovative year numbering. Concerning this year Dionusius contended that it was 525 years after the Incarnation of Christ, without giving any correspondence to the other dominant dating systems, such as number and year of Olympiad, year after the creation of the world (Anno Mundi), or reign year of Augustus. He himself stated that the "present year" was "the consulship of Probus Junior", which was 525 years "since the incarnation of our Lord Jesus Christ". Thus Dionysius implied that Jesus' incarnation occurred 525 years earlier.
As we mentioned the key work of Dionysius was the computation of correct Paschal (Easter) tables, continuing the tradition of Patriarchates of Alexandria (Cyril of Alexandria) and Rome (Victorius of Aquitaine). The Victorian tables were full of mistakes, so they were abandoned even by the Church of Rome, which followed the tradition of Alexandria and Constantinople. Dionysius used Cyrillian Easter tables, which ended on Diocletian year 247. Through the above mentioned correspondence, next year, Diocletian year 248, which was the first year of the next 5 decennovenal Metonic cycles (95 years), would be AD 532. This is a very peculiar coincidence, since the start year of the new Easter cycle was equal to the significant number 532, the product 19 (the period of the Metonic cycle) x 4 (the period of the leap bissextile years) x 7 (the period of the days of a week), which was also the period of the reappearance of the same Easter dates. Perhaps he had noticed this number in the Victorian Easter tables, but it is not certain that he had realized that it was really the Easter dates period. After this innovative dating correspondence, which is dominant throughout the world until now, all the dates and the Easter cycles were referred under the new format. The previous Easter table offered by Cyril concerned the years AD 437-531 and the next cycle offered by Dionysius concerned the years AD 532-626, later extended to the years AD 532-721. Years AD 532-626 were numbered as "Anni Domini Nostri Jesu Christi (Years of our Lord Jesus Christ)", as Dionysius explained to Petronius, because he did not wish to continue the memory of Diocletian, a tyrant who persecuted Christians.
The Anno Domini era became dominant in western Europe only after Venerable Bede, who used the Dionysius dating system to date the events in his 'Ecclesiastical History of the English People', completed in 731. Most of the British Church accepted the Dionysian tables after the Synod of Whitby in 664, which agreed to drop the old British method in favour of the Dionysius system. France accepted the new system during the late 8th century after the arrival of Alcuin from Britain.
Another interesting point of the arithmetic of Dionysius is the introduction of the concept of zero. This number, already used in India since AD 600, was absent from the European arithmetic, based on the ancient Greek and the Hellenistic (post-Babylonian) numbering. One of the necessary values for the estimation of the date of the Easter was the order of the year in the decennovenal Metonic cycle (see also Info item 8). To determine it, the Dionysian year plus one was divided by 19. If the result was zero (to be replaced by 19), it was represented by the Latin word 'nihil', also meaning nothing.