Anno Domini (Medieval Latin: In the year of (the/Our) Lord), abbreviated as AD or A.D., is a designation used to number years in the Julian and Gregorian calendars. More fully, years may be also specified as Anno Domini Nostri Iesu (Jesu) Christi ("In the Year of Our Lord Jesus Christ").
The calendar era which it numbers is based on the traditionally reckoned year of the conception or birth of Jesus. Before Christ, abbreviated as BC or B.C., is used in the English language to denote years before the start of this epoch.
Though the Anno Domini dating system was devised in 525, it was not until the 8th century that the system began to be adopted in Western Europe. According to the Catholic Encyclopedia, even popes continued to date documents according to regnal years, and usage of AD only gradually became more common in Europe from the 11th to the 14th centuries. In 1422, Portugal became the last Western European country to adopt the Anno Domini system.
Year numbering using the Anno Domini system (or its alternative Common Era (CE) designation) is the most widespread numbering system in the world today, including numbering of decades, centuries, and millennia. It is a de facto standard as used by international agencies such as the United Nations and the Universal Postal Union. Its preeminence is a consequence of the European colonisation of the other continents, thus spreading the Gregorian calendar.
Traditionally, English copied Latin usage by placing the abbreviation before the year number for AD, but after the year number for BC; for example: 64 BC, but AD . However, placing the AD after the year number (as in AD) is now also common. The abbreviation is also widely used after the number of a century or millennium, as in 4th century AD or 2nd millennium AD. In these cases it should be read as, e.g., "in the 4th century of the AD scale".
Because B.C. is an abbreviation for Before Christ, some people incorrectly conclude that A.D. must mean After Death, i.e., after the death of Jesus. If that were true, the thirty-three or so years of his life would not be in any era.
Although the last non-imperial consul, Basilius, was appointed in 541 by Emperor Justinian I, later emperors through Constans II (641–668) were appointed consuls on the first January 1 after their accession. All of these emperors, except Justinian, used imperial post-consular years for all of the years of their reign alongside their regnal years. Long unused, this practice was not formally abolished until Novell xciv of the law code of Leo VI did so in 888.
The Anno Domini system was devised by a monk named Dionysius Exiguus (born in Scythia Minor) in Rome in 525. In his Easter table Dionysius equates the year AD 532 with the regnal year 284 of Emperor Diocletian. In Argumentum I attached to this table he equates the year AD 525 with the consulate of Probus Junior. He thus implies that Jesus' Incarnation occurred 525 years earlier, without stating the specific year during which his birth or conception occurred.
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:
Two centuries later, the Anglo-Saxon historian Bede the Venerable used another Latin term, "ante uero incarnationis dominicae tempus" ("the time before the Lord's true incarnation"), equivalent to the English "before Christ", to identify years before the first year of this era.
Another calculation had been developed by the Alexandrian monk Annianus around the year AD 400, placing the Annunciation on March 25, AD 9 (Julian)—eight to ten years after the date that Dionysius was to imply. Although this Incarnation was popular during the early centuries of the Byzantine Empire, years numbered from it, an Era of Incarnation, was only used, and is still only used, in Ethiopia, accounting for the eight- or seven-year discrepancy between the Gregorian and the Ethiopian calendars. Byzantine chroniclers like Maximus the Confessor, George Syncellus and Theophanes dated their years from Annianus' Creation of the World. This era, called Anno Mundi, "year of the world" (abbreviated AM), by modern scholars, began its first year on 25 March 5492 BC. Later Byzantine chroniclers used Anno Mundi years from September 1 5509 BC, the Byzantine Era. No single Anno Mundi epoch was dominant throughout the Christian world.
The Gospel of St. Luke (1:5) states that St. John the Baptist was at least conceived, if not born, under King Herod, and that Jesus was conceived while St. John's mother St. Elizabeth was in the sixth month of her pregnancy (1:26). St. Luke's Gospel also states that Jesus was born during the reign of the Emperor Augustus and while Cyrenius (or Quirinius) was the governor of Syria (2:1–2). Blackburn and Holford-Strevens indicate Cyrenius/Quirinius' governorship of Syria began in AD 6, which is incompatible with conception in 4 BC, and say that "St. Luke raises greater difficulty....Most critics therefore discard Luke". Some scholars rely on St. John's Gospel to place Christ's birth in c. 18 BC.
On the continent of Europe, Anno Domini was introduced as the era of choice of the Carolingian Renaissance by Alcuin. This endorsement by Emperor Charlemagne and his successors popularizing the usage of the epoch and spreading it throughout the Carolingian Empire ultimately lies at the core of the system's prevalence until present times.
Outside the Carolingian Empire, Spain continued to date by the Era of the Caesars, or Spanish Era, which began counting from 38 BC, well into the Middle Ages,. The Era of Martyrs, which numbered years from the accession of Diocletian in 284, who launched the last yet most severe persecution of Christians, was used by the Church of Alexandria, and is still used officially by the Coptic church. It also used to be used by the Ethiopian church. Another system was to date from the crucifixion of Jesus Christ, which as early as Hippolytus and Tertullian was believed to have occurred in the consulate of the Gemini (AD 29), which appears in the occasional medieval manuscript. Most Syriac manuscripts written at the end of the 19th century still gave the date in the end-note using the "year of the Greeks" (Anno Graecorum = Seleucid era).
Even though Anno Domini was in widespread use by the 9th century, Before Christ (or its equivalent) did not become widespread until the late 15th century.