A calendar era is the year numbering system used by a calendar. For example, the Gregorian calendar numbers its years in the Western Christian era (the Coptic Orthodox and Ethiopian Orthodox churches have their own Christian eras). The instant, date, or year from which time is marked is called the epoch of the era. There are many different calendar eras such as Saka Era.
In antiquity, regnal years were counted from the accession of a monarch. This makes the Chronology of the ancient Near East very difficult to reconstruct, based on disparate and scattered king lists, such as the Sumerian King List and the Babylonian Canon of Kings. In East Asia, reckoning by era names chosen by ruling monarchs ceased in the 20th century except for Japan, where they are still used.
Ancient dating systems
For over a thousand years, ancient Assyria used a system of eponyms to identify each year. Each year at the Akitu festival (celebrating the Mesopotamian new year), one of a small group of high officials (including the king in later periods) would be chosen by lot to serve as the limmu for the year, which meant that he would preside over the Akitu festival and the year would bear his name. The earliest attested limmu eponyms are from the Assyrian trading colony at Karum Kanesh in Anatolia, dating to the very beginning of the 2nd millennium BC, and they continued in use until the end of the Neo-Assyrian Period, ca. 612 BC.
Assyrian scribes compiled limmu lists, including an unbroken sequence of almost 250 eponyms from the early 1st millennium BC. This is an invaluable chronological aid, because a solar eclipse was recorded as having taken place in the limmu of Bur-Sagale, governor of Guzana. Astronomers have identified this eclipse as one that took place on 15 June, 763 BC, which has allowed absolute dates of 892 to 648 BC to be assigned to that sequence of eponyms. This list of absolute dates has allowed many of the events of the Neo-Assyrian Period to be dated to a specific year, avoiding the chronological debates that characterize earlier periods of Mesopotamian history.
Among the ancient Greek historians and scholars, a common method of indicating the passage of years was based on the Olympic Games, first held in 776 BC. The Olympic Games provided the various independent city-states with a mutually recognizable system of dates. Olympiad dating was not used in everyday life. This system was in use from the 3rd century BC. The modern Olympic Games (or Summer Olympic Games beginning 1896) do not continue the four year periods from ancient Greece: the 669th Olympiad would have begun in the summer of 1897, but the modern Olympics were first held in 1896.
Another common system was the indiction cycle (15 indictions made up an agricultural tax cycle, an indiction being a year in duration). Documents and events began to be dated by the year of the cycle (e.g., "fifth indiction", "tenth indiction") in the 4th century, and this system was used long after the tax ceased to be collected. It was used in Gaul, in Egypt, and in most parts of Greece until the Islamic conquest, and in the Eastern Roman Empire until its conquest in 1453.
The rule for computing the indiction from the AD year number, which he had just invented, was stated by Dionysius Exiguus: add 3 and divide by 15; the remainder is the indiction, with 0 understood to be the fifteenth indiction. Thus 2001 was the ninth indiction. The beginning of the year varied.
The Seleucid era was used in much of the Middle East from the 4th century BC to the 6th century AD, and continued until the 10th century AD among Oriental Christians. The era is computed from the epoch 312 BC: in August of that year Seleucus I Nicator captured Babylon and began his reign over the Asian portions of Alexander the Great's empire. Thus depending on whether the calendar year is taken as starting on 1 Tishri or on 1 Nisan (respectively the start of the Jewish civil and ecclesiastical years) the Seleucid era begins either in 311 BC (the Jewish reckoning) or in 312 BC (the Greek reckoning: October–September).
An early and common practice was Roman 'consular' dating. This involved naming both consules ordinarii who had taken up this office on January 2 of the relevant civil year. Sometimes one or both consuls might not be appointed until November or December of the previous year, and news of the appointment may not have reached parts of the Roman empire for several months into the current year; thus we find the occasional inscription where the year is defined as "after the consulate" of a pair of consuls.
The use of consular dating ended in AD 541 when the emperor Justinian I discontinued appointing consuls. The last consul nominated was Anicius Faustus Albinus Basilius. Soon afterwards, imperial regnal dating was adopted in its place.
Dating from the founding of Rome
Another method of dating, rarely used, was anno urbis conditae (Latin: "in the year of the founded city" (abbreviated AUC), where "city" meant Rome). (It is often incorrectly given that AUC stands for ab urbe condita, which is the title of Titus Livius's history of Rome.)
Several epochs were in use by Roman historians. Modern historians usually adopt the epoch of Varro, which we place in 753 BC.
The system was introduced by Marcus Terentius Varro in the 1st century BC. The first day of its year was Founder's Day (April 21), although most modern historians assume that it coincides with the modern historical year (January 1 to December 31). It was rarely used in the Roman calendar and in the early Julian calendar – naming the two consuls that held office in a particular year was dominant. AD 2017 is thus approximately the same as AUC 2770 (2017 + 753).
Regnal years of Roman emperors
Another system that is less commonly found than might be thought was the use of the regnal year of the Roman emperor. At first, Augustus indicated the year of his reign by counting how many times he had held the office of consul, and how many times the Roman Senate had granted him Tribunican powers, carefully observing the fiction that his powers came from these offices granted to him, rather than from his own person or the many legions under his control. His successors followed his practice until the memory of the Roman Republic faded (about AD 200), when they began to use their regnal year openly.
Dating from the Roman conquest
Some regions of the Roman Empire dated their calendars from the date of Roman conquest, or the establishment of Roman rule.
The Spanish era counted the years from 38 BC, probably the date of a new tax imposed by the Roman Republic on the subdued population of Iberia. The date marked the establishment of Roman rule in Spain and was used in official documents in Portugal, Aragon, Valencia, and in Castile, into the 14th century. This system of calibrating years fell to disuse in 1381 and was replaced by today's Anno Domini.
Throughout the Roman and Byzantine periods, the Decapolis and other Hellenized cities of Syria and Palestine used the Pompeian era, counting dates from the Roman general Pompey's conquest of the region in 63 BC.
A different form of calendar was used to track longer periods of time, and for the inscription of calendar dates (i.e., identifying when one event occurred in relation to others). This form, known as the Long Count, is based upon the number of elapsed days since a mythological starting-point. According to the calibration between the Long Count and Western calendars accepted by the great majority of Maya researchers (known as the GMT correlation), this starting-point is equivalent to August 11, 3114 BC in the proleptic Gregorian calendar or 6 September in the Julian calendar (−3113 astronomical).
Other dating systems
A great many local systems or eras were also important, for example the year from the foundation of one particular city, the regnal year of the neighboring Persian emperor, and eventually even the year of the reigning Caliph.
Late Antiquity and Middle Ages
- The Etos Kosmou of the Byzantine Calendar places Creation at the beginning of its year 1, namely 5509 BC. Its first known use occurred in the 7th century AD, although its precursors were developed about AD 400. The year 7509 of this era began in September 2000.
- The Era of Martyrs or Era of Diocletian is reckoned from the beginning of the reign of Roman Emperor Diocletian; the first year of this era was 284/5. It was not the custom to use regnal years in Rome, but it was the custom in Roman Egypt, which the emperor ruled through a prefect (the king of Egypt). The year number changed on the first day of the Egyptian month Thoth (29 August three years out of four, 30 August the year before a Roman leap year.) Diocletian abolished the special status of Egypt, which thereafter followed the normal Roman calendar: consular years beginning on 1 January. This era was used in the Easter tables prepared in Alexandria long after the abdication of Diocletian, even though Diocletian was a notorious persecutor of Christians. The Era of Diocletian was retained by the Coptic Church and used for general purposes, but by 643 the name had been changed to Era of the Martyrs.
- The Incarnation Era is used by Ethiopia. Its epoch is August 29, AD 8 in the Julian calendar.
- The Armenian calendar has its era fixed at AD 552.
Dionysian "Common Era"
The era based on the Incarnation of Christ was introduced by Dionysius Exiguus in 525 and is in continued use with various reforms and derivations. The distinction between the Incarnation being the conception or the Nativity of Jesus was not drawn until the late ninth century. The beginning of the numbered year varied from place to place: when, in 1600, Scotland adopted January 1 as the date the year number changes, this was already the case in much of continental Europe. England adopted this practice in 1752.
- A.D. (or AD) – for the Latin Anno Domini, meaning "in the year of (our) Lord". This is the dominant or Western Christian Era; AD is used in the Gregorian calendar. Anno Salutis, meaning "in the year of salvation" is identical. Originally intended to number years from the Incarnation of Jesus, according to modern thinking the calculation was a few years off. Years preceding AD 1 are numbered using the BC era, avoiding zero or negative numbers. AD was also used in the medieval Julian calendar, but the first day of the year was either March 1, Easter, March 25, September 1, or December 25, not January 1. To distinguish between the Julian and Gregorian calendars, O.S. and N.S. were often added to the date, especially during the 17th and 18th centuries, when both calendars were in common use. Old Style (O.S.) was used for the Julian calendar and for years not beginning on January 1. New Style (N.S.) was used for the Gregorian calendar and for Julian calendar years beginning on January 1. Many countries switched to using January 1 as the start of the numbered year at the same time as they switched from the Julian calendar to the Gregorian calendar, but others switched earlier or later.
- B.C. (or BC) – meaning "Before Christ". Used for years before AD 1, counting backwards so the year n BC is n years before AD 1. Thus there is no year 0.
- C.E. (or CE) – meaning "Common Era", numerically equivalent to Anno Domini. (In writing, "AD" precedes the year number, but "CE" follows the year: AD 1 = 1 CE.) The first use of the Latin equivalent (vulgaris aera) discovered so far was in a 1615 book by Johannes Kepler.
- B.C.E. (or BCE) – meaning "Before the Common Era". Equivalent to B.C. (1 BC = 1 BCE.)
- Astronomical year numbering equates its year 0 with 1 BC, and counts negative years from 2 BC backward (−1 backward), so 100 BC is −99.
- The human era, also named Holocene era, proposed by Cesare Emiliani adds 10,000 to AD years, so that AD 1 would be the year 10,001.
- Anno Lucis of Freemasonry adds 4000 years to the AD year.
- A.H. (or AH) – for the Latinized Anno Hegirae, meaning "in the year of the Hijra", Muhammad's flight from Mecca to Medina in September 622, which occurred in its first year, used in the Islamic calendar. Since the Islamic calendar is a purely lunar calendar of about 354 days, its year count increases faster than that of solar and lunisolar calendars.
- A.H.S. (or AHS) is used by the Iranian calendar to denote the number of solar years since the Hijra. The year beginning at the vernal equinox equals the number of the Gregorian year beginning at the preceding January 1 minus 621.
- Hindu calendar, counting from the start of the Kali Yuga, with its epoch on February 18, 3102 BC Julian (January 23, 3102 BC Gregorian), based on Aryabhata (6th century).
- Vikrama Samvat, 56-57 BC, introduced about the 12th century.
- S.E. or (SE) – for the Saka Era, used in some Hindu calendars and in the Indian national calendar, with an epoch near the vernal equinox of year 78 (its year 0); its usage spread to Southeast Asia before year 1000. This era is also used (together with the Gregorian calendar) in the Indian national calendar, the official civil calendar used in communiques issued by the Government of India.
The Hindu Saka Era influences the calendars of southeast Asian indianized kingdoms.
- B.E. – for the Buddhist Era, introduced by Vajiravudh in 1912, which has an epoch (origin) of 544 BC. This year is called year 1 in Sri Lanka and Burma, but year 0 in Thailand, Laos and Cambodia. Thus the year 2500 B.E. occurred in 1956 in the former countries, but in 1957 in the latter. In Thailand in 1888 King Chulalongkorn decreed a National Thai Era, dating from the founding of Bangkok on April 6, 1782. In 1912 New Year's Day was shifted to April 1. In 1941 Prime Minister Phibunsongkhram decided to count the years since 543 BC. This is the Thai solar calendar using the Thai Buddhist Era aligned to the western solar calendar.
- BE for Burmese Era – from Burmese calendar originally with an epochal year 0 date of 22 March 638; from which derived CS for Chula Sakarat era; variously known as LE Lesser Era; ME Minor Era – the Major or Great Era being the Saka Era of the Indian national calendar
- B.E. of the Bahá'í calendar is below.
- B.E. – The Bahá'í calendar dates from the year of the declaration of the Báb. Years are counted in the Bahá'í Era (BE), which starts its year 1 from March 21, 1844.
- A.M. (or AM) – for the Latin Anno Mundi, meaning "in the year of the world", has its epoch in the year 3761 BC. This was first used to number the years of the modern Hebrew calendar in 1178 by Maimonides. Precursors with epochs one or two years later were used since the 3rd century, all based on the Seder Olam Rabba of the 2nd century. The year beginning in the northern autumn of 2000 was 5761 AM).
- The Zoroastrian calendar used regnal years since the reform by Ardeshir I, but after the fall of the Sassanid empire, the ascension of the last Sassanid ruler, Yazdegerd III of Persia, crowned June 16, 632, continued to be used as the reference year, abbreviated Y.Z. or "Yazdegerd era".
- The Republican Era of the French Republican Calendar was dated from September 22, 1792, the day of the proclamation of the French First Republic. It was used in Revolutionary France from October 24, 1793 (on the Gregorian calendar) to December 31, 1805.
- The Positivist calendar of 1844 takes 1789 as its epoch.
- The Republican era is used by the Republic of China since 1912, which is the first year of the republic. Coincidentally, this is the same as the Juche era used in North Korea, the year of the birth of its founder Kim Il-Sung.
- The Italian Fascists used Roman numerals to denote the number of years since the March on Rome in 1922. Therefore, 1934, for example, was XII E.F. (era fascista). This era was abolished with the fall of fascism in Italy on July 25, 1943, but restored in the northern part of the country during the Italian Social Republic. The Gregorian calendar remained in simultaneous use and a double numbering was adopted: the year of the Common era was presented in Arabic numerals and the year of the fascist era in Roman numerals. The year of the Fascist calendar began on October 29, so, for example, October 27, 1933 was XI E.F. but October 30, 1933 was XII E.F.
- China traditionally reckoned by the regnal year of its emperors, see Chinese era name. Most Chinese do not assign numbers to the years of the Chinese calendar, but the few who do, like expatriate Chinese, use a continuous count of years from the reign of the legendary Yellow Emperor, using 2698 BC as year 1. Western writers begin this count at either 2637 BC or 2697 BC (see Chinese calendar). Thus, the Chinese years 4637, 4697, or 4698 began in early 2000.
- In Korea, from 1952 until 1961 years were numbered via Dangi years, where 2333 BC was regarded as the first such year.
- The Assyrian calendar, introduced in the 1950s, has its era fixed at 4750 BC.
- The Japanese calendar dates from the accession of the current Emperor of Japan. The current emperor took the throne in early 1989, which became Heisei 1, which was until then Shōwa 64 (for its first seven days).
- The United States government has at times used parallel calendar eras. For instance, its Constitution marked its date as being "the Seventeenth Day of September in the Year of our Lord one thousand seven hundred and Eighty seven and of the Independance of the United States of America the Twelfth".
- A.D. – "After Dianetics". In Scientology, years are numbered relative to the first publication of the book Dianetics: The Modern Science of Mental Health (1950).
- Y.O.L.D. – In the Discordian calendar, the standard designation for the year number is YOLD (Year of Our Lady of Discord). The calendar begins counting from January 1, 1166 BC in the Discordian year 0, ostensibly the date of origin of the Curse of Greyface. An alternate designation, A.D.D. has been occasionally seen (Anno Domina Discordia, a Latin translation of YOLD, but presumably also a play on attention deficit disorder).
- e.v. – Era vulgaris. (From Latin, meaning "common era", usually stylized in lowercase.) The Thelemic calendar is used by some Thelemites to designate a number of years since Aleister Crowley's inauguration of the so-called Aeon of Horus, which occurred on March 20, 1904, and coincides with both the Thelemic new year and a holiday known as the Equinox of the Gods. The abbreviation "A.N.", for Aerae Novae ("New Era" in Latin), is sometimes also used.
- B.P. – for Before Present, specifically, the number of radiocarbon years before 1950.
- HE – for counting elapsed years of the Holocene from near the beginning of the Neolithic revolution of the Holocene epoch, specifically by adding exactly 10,000 years to AD (Anno Domini) or CE (Common Era) years, and subtracting BC/BCE years from 10001.
- Julian day number – for counting days, not years, its era fixed at noon January 1, 4713 BC in the proleptic Julian calendar. This equals November 24, 4714 BC in the proleptic Gregorian calendar. From noon of this day to noon of the next day was day 0. Multiples of 7 are Mondays. Negative values can also be used. Apart from the choice of the zero point and name, this Julian day and Julian date are not related to the Julian calendar. It does not count years, so, strictly speaking, it has no era, but it does have an epoch. Today (noon-to-noon UTC) the value is 2458094.
- Unix time – for counting elapsed seconds since the Unix epoch set at 00:00:00 or midnight UTC of January 1, 1970, though there are problems with Unix implementation of Coordinated Universal Time (UTC).