The Anno Domini dating system was devised in 525 by Dionysius Exiguus 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."
Bonnie J. Blackburn and Leofranc 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 (30 August in the year before a Julian leap year).
- There were inaccuracies in the lists of consuls.
- There were confused summations of emperors' regnal years.
It is not known how Dionysius established the year of Jesus's birth. Two major theories are that Dionysius based his calculation on the Gospel of Luke, which states that Jesus was "about thirty years old" shortly after "the fifteenth year of the reign of Tiberius Caesar", and hence subtracted thirty years from that date, or that Dionysius counted back 532 years from the first year of his new table. It is convenient to initiate a calendar not from the very day of an event but from the beginning of a cycle which occurs in close proximity. For example, the Islamic calendar begins not from the date of the Hijra, but rather weeks prior, on the first occurrence of the month of Muharram (corresponding to 16 July 622).
It has also been speculated by Georges Declercq that Dionysius' desire to replace Diocletian years with a calendar based on the incarnation of Christ was intended to prevent people from believing the imminent end of the world. At the time, it was believed by some that the resurrection of the dead 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 and the end of the world but this date had already passed in the time of Dionysius.
The Anglo-Saxon historian the Venerable Bede, who was familiar with the work of Dionysius Exiguus, used Anno Domini dating in his Ecclesiastical History of the English People, completed in 731. In this same history, he also used another Latin term, ante vero incarnationis dominicae tempus anno sexagesimo ("in fact in the 60th year before the time of the Lord's incarnation"), equivalent to the English "before Christ", to identify years before the first year of this era. Both Dionysius and Bede regarded Anno Domini as beginning at the incarnation of Jesus, but "the distinction between Incarnation and Nativity was not drawn until the late 9th century, when in some places the Incarnation epoch was identified with Christ's conception, i.e., the Annunciation on March 25" (Annunciation style).
On the continent of Europe, Anno Domini was introduced as the era of choice of the Carolingian Renaissance by the English cleric and scholar Alcuin in the late eighth century. Its endorsement by Emperor Charlemagne and his successors popularizing the use of the epoch and spreading it throughout the Carolingian Empire ultimately lies at the core of the system's prevalence. According to the Catholic Encyclopedia, popes continued to date documents according to regnal years for some time, but usage of AD gradually became more common in Roman Catholic countries from the 11th to the 14th centuries. In 1422, Portugal became the last Western European country to switch to the system begun by Dionysius. Eastern Orthodox countries only began to adopt AD instead of the Byzantine calendar in 1700 when Russia did so, with others adopting it in the 19th and 20th centuries.
Although Anno Domini was in widespread use by the 9th century, the term "Before Christ" (or its equivalent) did not become common until much later. Bede used the expression "anno igitur ante incarnationem Dominicam" (so in the year before the incarnation of the Lord) twice. "Anno an xpi nativitate" (in the year before the birth of Christ) is found in 1474 in a work by a German monk. In 1627, the French Jesuit theologian Denis Pétau (Dionysius Petavius in Latin), with his work De doctrina temporum, popularized the usage ante Christum (Latin for "Before Christ") to mark years prior to AD.
When the reckoning from Jesus' incarnation began replacing the previous dating systems in western Europe, various people chose different Christian feast days to begin the year: Christmas, Annunciation, or Easter. Thus, depending on the time and place, the year number changed on different days in the year, which created slightly different styles in chronology:
- From 25 March 753 AUC (today in 1 BC), i.e., notionally from the incarnation of Jesus. That first "Annunciation style" appeared in Arles at the end of the 9th century, then spread to Burgundy and northern Italy. It was not commonly used and was called calculus pisanus since it was adopted in Pisa and survived there till 1750.
- From 25 December 753 AUC (today in 1 BC), i.e., notionally from the birth of Jesus. It was called "Nativity style" and had been spread by Bede together with the Anno Domini in the early Middle Ages. That reckoning of the Year of Grace from Christmas was used in France, England and most of western Europe (except Spain) until the 12th century (when it was replaced by Annunciation style), and in Germany until the second quarter of the 13th century.
- From 25 March 754 AUC (today in AD 1). That second "Annunciation style" may have originated in Fleury Abbey in the early 11th century, but it was spread by the Cistercians. Florence adopted that style in opposition to that of Pisa, so it got the name of calculus florentinus. It soon spread in France and also in England where it became common in the late 12th century and lasted until 1752.
- From Easter, starting in 754 AUC (AD 1). That mos gallicanus (French custom) bound to a moveable feast was introduced in France by king Philip Augustus (r. 1180–1223), maybe to establish a new style in the provinces reconquered from England. However, it never spread beyond the ruling élite.
With these various styles, the same day could, in some cases, be dated in 1099, 1100 or 1101.