Anno Domini

• 1. Introduction

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  Anno Domini, Medieval Latin for "in the year of the Lord", is the phrase behind the abbreviation AD. It counts years forward from an epoch, the traditionally reckoned year of the conception or birth of Jesus. Its partner, BC, counts backward from that same point. The abbreviation BC belongs to English alone, and other languages use their own equivalents. The Latin form, ante Christum natum, is rarely seen in English. Together these labels qualify years in both the Julian and Gregorian calendars. The man who invented the scheme left a puzzle behind him. Why did he build it, and from what exact year did he count? How did it travel from a single table of calculations to the habit of a continent? And why did the year number once change on different days depending on where a person lived?

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• 2. Dionysius and Easter

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  In 525, in Rome, an Eastern Roman monk named Dionysius Exiguus devised a way of numbering years for his Easter table. Older Easter tables ran on the Diocletian era. Dionysius refused to continue it, because he did not want to keep alive the memory of a tyrant who persecuted Christians.

  The join between the two systems was exact. The last year of the old table was Diocletian Anno Martyrium 247. The very next year was the first of his own, anno Domini 532.

  At the time, Julian calendar years were identified by naming the consuls in office. Dionysius described his present year as "the consulship of Probus Junior". He then said it stood 525 years "since the incarnation of our Lord Jesus Christ". That wording implies the incarnation happened 525 years earlier, but it never names the year of the birth or the conception.

  Dionysius also never connected his epoch to any consulate, Olympiad, year of the world or regnal year of Augustus. Nor did he explain or defend the date underneath his system.

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• 3. A Disputed Epoch

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  Bonnie J. Blackburn and Leofranc Holford-Strevens lay out arguments for three different years as the one Dionysius meant: 2 BC, 1 BC or AD 1. Several sources of confusion explain the spread. Bede treated the incarnation as the same thing as the Nativity, while modern usage equates it with conception. The civil or consular year began on the 1st of January, but the Diocletian year began on the 1st of September. Lists of consuls contained errors, and summations of emperors' regnal years were muddled.

  Nobody knows how Dionysius reached his figure. One theory points to the Gospel of Luke, which says Jesus was "about thirty years old" shortly after "the fifteenth year of the reign of Tiberius Caesar". Subtracting thirty years from that date gives the answer. Tertullian, Eusebius and Epiphanius all agree on 2 BC, and probably reasoned this way, subtracting thirty from AD 29.

  Another major theory runs through Victorius of Aquitaine. His astronomical computations established a 532-year cycle, after which the dates for Easter repeat. On this view, Dionysius counted back 532 years from the first year of his new table. A third possibility is that he drew on an earlier, unknown source. The Chronograph of 354 already stated that Jesus was born in the consulship of Caesar and Paullus, which corresponds to AD 1.

  Modern scholars believe the actual birth came about 5 BC. The gospels and secular texts give no date, and most scholars assume somewhere between 6 BC and 4 BC. The evidence is too fragmentary for a firm answer, so they estimate in two ways. One analyzes references to known historical events in the Nativity accounts of Luke and Matthew. The other works backward from the estimated start of the ministry of Jesus.

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• 4. Fear of the Year 6000

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  The Anno Mundi calendar counted from the creation of the world, as described in the Old Testament, and it placed the birth of Jesus in the year 5500. Under that system, the year 6000 would mark the end of the world. Anno Mundi 6000 falls at approximately AD 500, and in the time of Dionysius that date had already passed. Some people had also believed that the resurrection of the dead and the end of the world would come 500 years after the birth of Jesus.

  Georges Declercq has speculated that this anxiety shaped the monk's project. On his reading, Dionysius wanted a calendar based on the incarnation of Christ in place of the Diocletian years. The aim was to stop people believing that the end of the world was imminent. This is a speculation, not a documented motive.

  Anno Mundi itself would not stay in one form. Byzantine chroniclers later built their own versions of it, with different starting points.

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• 5. Rival Ways of Counting

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  Basilius was appointed consul in 541 by Emperor Justinian I, and he was the last consul who was not an emperor. Later emperors, through Constans II, who reigned 641-668, took the consulship on the 1st of January after they came to power. Every one of them except Justinian counted imperial post-consular years alongside regnal years. The practice lay unused for a long time and was formally abolished only in 888, by Novell XCIV of the law code of Leo VI.

  Around the year AD 400, an Alexandrian monk named Annianus developed another calculation. It placed the Annunciation on the 25th of March AD 9 in the Julian calendar, eight to ten years after the date Dionysius would later imply. Years numbered from it, an Era of Incarnation, were popular in the early centuries of the Byzantine Empire. In Ethiopia they were used exclusively, and they still are. That accounts for the seven- or eight-year gap between the Ethiopian and Gregorian calendars.

  Byzantine chroniclers such as Maximus the Confessor, George Syncellus and Theophanes dated their years from Annianus' creation of the world. Modern scholars call this era Anno Mundi, abbreviated AM. Its first year began on the 25th of March 5492 BC. Later Byzantine chroniclers started instead from the 1st of September 5509 BC, the Byzantine Era. No single Anno Mundi epoch dominated the Christian world. Eusebius of Caesarea, in his Chronicle, counted from the birth of Abraham, dated 2016 BC, so that AD 1 equaled 2017 Anno Abrahami.

  Spain and Portugal kept using the Spanish Era, also called the Era of the Caesars, which counted from 38 BC. It lasted well into the Middle Ages. The Era of Martyrs numbered years from the accession of Diocletian in 284. The Church of Alexandria used it, as did the Ethiopian and Eritrean churches. The Coptic Orthodox and Coptic Catholic churches still use it officially. Another approach dated from the crucifixion, which Hippolytus and Tertullian already believed to have occurred in the consulate of the Gemini, AD 29. That reckoning appears in some medieval manuscripts.

  In the 9th century, the Historia Brittonum attributed to Nennius leaned heavily on Anno Passionis, counted from the Year of the Passion, alongside the newer AD. Experts generally accept a 27-year difference between the two.

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• 6. Carried by Scholars and Kings

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  The Anglo-Saxon historian Bede completed his Ecclesiastical History of the English People in AD 731. He knew the work of Dionysius Exiguus and used anno Domini dating in it. He also wrote ante … incarnationis dominicae tempus anno sexagesimo, meaning "in the sixtieth year before the time of the Lord's incarnation". That phrase is the equivalent of the English "before Christ".

  On the continent of Europe, the English cleric and scholar Alcuin introduced anno Domini in the late eighth century. It became the era of choice of the Carolingian Renaissance. Emperor Charlemagne and his successors endorsed it, and that backing lies at the core of the system's prevalence across the Carolingian Empire.

  By the 9th century the system was in widespread use. Popes kept dating documents by regnal years for some time. From the 11th to the 14th centuries, AD gradually grew more common in Catholic countries. In 1422, Portugal became the last Western European country to switch. Eastern Orthodox countries began adopting AD instead of the Byzantine calendar only in 1700, when Russia did so. Others followed in the 19th and 20th centuries.

  The label for years before the epoch lagged far behind. A German monk wrote "Anno ante Christi nativitatem", meaning "in the year before the birth of Christ", in a work of 1474. In 1627, the French Jesuit theologian Denis Pétau, known in Latin as Dionysius Petavius, popularized ante Christum in his work De doctrina temporum.

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• 7. When the Year Began

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  In Pisa, the year number rolled over on the 25th of March, and the city kept that habit until 1750. This "Annunciation style" counted notionally from the incarnation of Jesus, nine months before Christmas. It first appeared in Arles at the end of the 9th century, then spread to Burgundy and northern Italy. It was not commonly used, and it took the name calculus pisanus, the Pisan calculation, from the city that adopted it.

  The distinction between the Incarnation and the Nativity was not drawn until the late 9th century. Only then, in some places, was the Incarnation epoch identified with Christ's conception, the Annunciation on the 25th of March. Before that, Bede had spread "Nativity style" together with anno Domini. Its year began on the 25th of December. France, England and most of western Europe except Spain used it until the 12th century, when Annunciation style replaced it. Germany kept it until the second quarter of the 13th century.

  A second Annunciation style began the year on the 25th of March, but one year later in the count. It may have originated in Fleury Abbey in the early 11th century, and the Cistercians spread it. Florence adopted it in opposition to Pisa, which gave it the name calculus florentinus. It soon spread in France. In England it became common in the late 12th century and lasted until 1752.

  A fourth custom tied the new year to Easter, a moveable feast. This mos gallicanus, the French custom, was introduced in France by King Philip Augustus, who reigned 1180-1223. He may have wanted to establish a new style in provinces reconquered from England. It never spread beyond the ruling elite.

  With so many styles in circulation, the same day could in some cases be dated 1099-1100 or 1101.

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• 8. Naming and Numbering

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  The abbreviation BC sits after the year number, as in 70 BC, while AD traditionally sits before it, as in AD 70. That order preserves normal syntax. AD is also widely used after a century or millennium, as in "fourth century AD", though conservative usage once rejected such phrases. Because BC stands for Before Christ, some people wrongly conclude that AD means After Death. That reading would leave the roughly 33 years associated with the life of Jesus in neither BC nor AD.

  Other names for the era appeared over time. The Latin vulgaris aerae is found in 1615. "Vulgar Era" appears in English as early as 1635, "Christian Era" in 1652 and "Common Era" in 1708. "Current Era" is another variant. The terms Common Era and Current Era, abbreviated CE, keep the same epoch while avoiding explicit religious references. Cunningham and Starr wrote in 1998 that B.C.E. and C.E. "do not presuppose faith in Christ and hence are more appropriate for interfaith dialog than the conventional B.C./A.D."

  China took its own path. When the Republic of China was founded, it adopted the Minguo Era but used the Western calendar for international purposes, under the translated term 西元 (xī yuán), meaning Western Era. In 1949, the People's Republic of China adopted 公元 (gōngyuán), meaning Common Era, for all purposes at home and abroad.

  The year 1 BC is followed directly by AD 1, because the system has no year zero. Most experts agree that new decades, centuries and millennia begin on a year ending in one, though the general public is divided. For computational reasons, astronomical year numbering and ISO 8601 make AD 1 year 1, 1 BC year 0 and 2 BC year -1. The standards differ in other ways too. ISO 8601 uses the Gregorian calendar, while ancient dates are usually expressed in the Julian calendar. A date that uses year 0 or a negative year therefore needs further investigation before anyone converts it to BC or AD.

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