Anno Domini - AD

The Anno Domini dating system was devised in 525 by Dionysius to enumerate the years in his Easter table. His system was to replace the Diocletian era that had been used in an old Easter table because he did not wish to continue the memory of a tyrant who persecuted Christians.

The last year of the old table, Diocletian 247, was immediately followed by the first year of his table, AD 532. When he devised his table, Julian calendar years were identified by naming the consuls who held office that year—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, without stating the specific year during which his birth or conception occurred.

However, nowhere in his exposition of his table does Dionysius relate his epoch to any other dating system, whether consulate, Olympiad, year of the world, or regnal year of Augustus; much less does he explain or justify the underlying date:778

Blackburn & Holford-Strevens briefly present arguments for 2 BC, 1 BC, or AD 1 as the year Dionysius intended for the Nativity or Incarnation. Among the sources of confusion are:

• In modern times Incarnation is synonymous with the conception, but some ancient writers, such as Bede, considered Incarnation to be synonymous with the Nativity
• The civil, or consular year began on 1 January but the Diocletian year began on 29 August
• There were inaccuracies in the list of consuls
• There were confused summations of emperors' regnal years

It has also been speculated by Georges Declercq that Dionysius' desire to replace Diocletian years (Diocletian persecuted Christians) with a calendar based on the incarnation of Christ was to prevent people from believing the imminent end of the world. At the time it was believed that the Resurrection and end of the world would occur 500 years after the birth of Jesus. The old Anno Mundi calendar theoretically commenced with the creation of the world based on information in the Old Testament. It was believed that based on the Anno Mundi calendar Jesus was born in the year 5500 (or 5500 years after the world was created) with the year 6000 of the Anno Mundi calendar marking the end of the world. Anno Mundi 6000 (approximately AD 500) was thus equated with the resurrection of Christ and the end of the world but this date had already passed in the time of Dionysius. Dionysius therefore searched for a new end of the world at a later date. He was heavily influenced by ancient cosmology, in particular the doctrine of the Great Year that places a strong emphasis on planetary conjunctions. Dionysius decided that when all the planets were in conjunction that this cosmic event would mark the end of the world. Dionysius accurately calculated that this conjunction would occur in May AD 2000, about 1500 years after the life of Dionysius. Dionysius then applied another cosmological timing mechanism based on precession of the equinoxes (that had only been discovered about six centuries earlier). Though incorrect, many people at the time believed that the precessional cycle was 24,000 years which included twelve astrological ages of 2,000 years each. Dionysius believed that if the planetary alignment of May 2000 marked the end of an age, then the birth of Jesus Christ marked the beginning of the age 2,000 years earlier on the 23rd March (the date of the Northern Hemisphere Spring Equinox and beginning of many yearly calendars from ancient times). He therefore deducted 2,000 years from the May 2000 conjunction to produce AD 1 for the incarnation of Christ even though modern scholars and the Roman Catholic Church acknowledge that the birth of Jesus was a few years earlier than AD 1.