The terms anno Domini (AD) and before Christ (BC)[note 1] are used to label or number years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin and means "in the year of the Lord", but is often presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ".
This calendar era is based on the traditionally reckoned year of the conception or birth of Jesus, with AD counting years from the start of this epoch and BC denoting years before the start of the era. There is no year zero in this scheme; thus the year AD 1 immediately follows the year 1 BC. This dating system was devised in 525 by Dionysius Exiguus of Scythia Minor, but was not widely used until the 9th century. 
Traditionally, English follows Latin usage by placing the "AD" abbreviation before the year number, though it is also found after that year. In contrast, BC is always placed after the year number (for example: AD 2021, but 68 BC), which preserves syntactic order. The abbreviation AD is also widely used after the number of a century or millennium, as in "fourth century AD" or "second millennium AD" (although conservative usage formerly rejected such expressions). Because BC is the English abbreviation for Before Christ, it is sometimes incorrectly concluded that AD means After Death, i.e., after the death of Jesus. However, this would mean that the approximate 33 years commonly associated with the life of Jesus would be included in neither the BC nor the AD time scales.
Terminology that is viewed by some as being more neutral and inclusive of non-Christian people is to call this the Current or Common Era (abbreviated as CE), with the preceding years referred to as Before the Common or Current Era (BCE). Astronomical year numbering and ISO 8601 avoid words or abbreviations related to Christianity, but use the same numbers for AD years (but not for BC years in the case of astronomical years; e.g., 1 BC is year 0, 45 BC is year −44).
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, as he did not wish to continue the memory of a tyrant who persecuted Christians. The last year of the old table, Diocletian Anno Martyrium 247, was immediately followed by the first year of his table, Anno Domini 532. When Dionysius devised his table, Julian calendar years were identified by naming the consuls who held office that year— Dionysius 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:
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 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 (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 end of the world but this date had already passed in the time of Dionysius. The "Historia Brittonum" attributed to Nennius written in the 9th century makes extensive use of the Anno Passionis (AP) dating system which was in common use as well as the newer AD dating system. The AP dating system took its start from 'The Year of The Passion'. It is generally accepted by experts there is a 27 year difference between AP and AD reference.
The Anglo-Saxon historian Saint (Venerable) Bede, who was familiar with the work of Dionysius Exiguus, used Anno Domini dating in his Ecclesiastical History of the English People, which he completed in AD 731. In the History he also used the Latin phrase ante [...] incarnationis dominicae tempus anno sexagesimo ("in the sixtieth year before the time of the Lord's incarnation"), which is 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 Christ, 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" dating).
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 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 [...] ante incarnationem Dominicam" (in the year before the incarnation of the Lord) twice. "Anno ante Christi nativitatem" (in the year before the birth of Christ) is found in 1474 in a work by a German monk.[note 2] 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:
With these various styles, the same day could, in some cases, be dated in 1099, 1100 or 1101.
The date of birth of Jesus of Nazareth is not stated in the gospels or in any secular text, but most scholars assume a date of birth between 6 BC and 4 BC. The historical evidence is too fragmentary to allow a definitive dating, but the date is estimated through two different approaches—one by analyzing references to known historical events mentioned in the Nativity accounts in the Gospels of Luke and Matthew and the second by working backwards from the estimation of the start of the ministry of Jesus.
During the first six centuries of what would come to be known as the Christian era, European countries used various systems to count years. Systems in use included consular dating, imperial regnal year dating, and Creation dating.
Although the last non-imperial consul, Basilius, was appointed in 541 by Emperor Justinian I, later emperors through to Constans II (641–668) were appointed consuls on the first of January after their accession. All of these emperors, except Justinian, used imperial post-consular years for the years of their reign, along with 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.
Another calculation had been developed by the Alexandrian monk Annianus around the year AD 400, placing the Annunciation on 25 March 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, were exclusively used and are still used in Ethiopia. This accounts for the seven- or eight-year discrepancy between the Gregorian and 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 1 September 5509 BC, the Byzantine Era. No single Anno Mundi epoch was dominant throughout the Christian world. Eusebius of Caesarea in his Chronicle used an era beginning with the birth of Abraham, dated in 2016 BC (AD 1 = 2017 Anno Abrahami).
Spain and Portugal continued to date by the Spanish Era (also called Era of the Caesars), which began counting from 38 BC, well into the Middle Ages. In 1422, Portugal became the last Catholic country to adopt the Anno Domini system.
The Era of Martyrs, which numbered years from the accession of Diocletian in 284, who launched the most severe persecution of Christians, was used by the Church of Alexandria and is still used, officially, by the Coptic Orthodox and Coptic Catholic churches. It was also used by the Ethiopian church. Another system was to date from the crucifixion of Jesus, which as early as Hippolytus and Tertullian was believed to have occurred in the consulate of the Gemini (AD 29), which appears in some medieval manuscripts.
Alternative names for the Anno Domini era include vulgaris aerae (found 1615 in Latin), "Vulgar Era" (in English, as early as 1635), "Christian Era" (in English, in 1652), "Common Era" (in English, 1708), and "Current Era". Since 1856, the alternative abbreviations CE and BCE (sometimes written C.E. and B.C.E.) are sometimes used in place of AD and BC.
The "Common/Current Era" ("CE") terminology is often preferred by those who desire a term that does not explicitly make religious references. For example, Cunningham and Starr (1998) write that "B.C.E./C.E. […] do not presuppose faith in Christ and hence are more appropriate for interfaith dialog than the conventional B.C./A.D." Upon its foundation, the Republic of China adopted the Minguo Era but used the Western calendar for international purposes. The translated term was 西元 (xī yuán; 'Western Era'). Later, in 1949, the People's Republic of China adopted 公元 (gōngyuán; 'Common Era') for all purposes domestic and foreign.
In the AD year numbering system, whether applied to the Julian or Gregorian calendars, AD 1 is immediately preceded by 1 BC, with nothing in between them (there was no year zero). There are debates as to whether a new decade, century, or millennium begins on a year ending in zero or one.
For computational reasons, astronomical year numbering and the ISO 8601 standard designate years so that AD 1 = year 1, 1 BC = year 0, 2 BC = year −1, etc.[note 3] In common usage, ancient dates are expressed in the Julian calendar, but ISO 8601 uses the Gregorian calendar and astronomers may use a variety of time scales depending on the application. Thus dates using the year 0 or negative years may require further investigation before being converted to BC or AD.