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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 and Ethiopic churches have their own Christian eras, see below). The instant, date, or year from which time is marked is called the epoch of the era. There are many different calendar eras.
In antiquity, regnal years were counted from the ascension 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 or the Babylonian Canon of Kings. In East Asia, reckoning by era names appointed by ruling monarchs remained current down to the 20th century (in Japan, down to the present day).
The first "universal" calendar era not dependent on the ruling monarch was introduced in the 4th century BC, with the olympiad, counted from the first Olympic Games in 776 BC.
- Olympiads, fixed at 776 BC, in use from the 4th c. BC down to the 3rd or 4th century AD; the "modern olympiad" is set at 1896.
- The Seleucid era, formerly used in much of the Middle East, uses the epoch 312 BC, the year when Seleucus I Nicator captured Babylon and began his reign over the Asian portions of Alexander the Great's empire. The era was in use from the 3rd century BC down to the 6th century AD.
- A.U.C. (or AUC) — for the Latin ab urbe condita, meaning from the founding of the city (of Rome), was introduced by Marcus Terentius Varro in the 1st century BC, although this was probably calculated inaccurately. 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. Dionysius Exiguus implied, but did not explicitly state, that AD 1 was 754 AUC, so that the year 2021 is the same as the year 2774 AUC (2021 + 753).
Late Antiquity and Middle Ages
Most of the traditional calendar eras in use today were introduced at the time of transition from Late Antiquity to the Early Middle Ages, roughly between the 6th and 10th centuries.
- Aetos Kosmou of the Byzantine Greek Calendar claims to date from Creation, introduced in ca. AD 400. The year 7516 of this era begins in September 2007 CE.
- A.M. (or AM), Era of Martyrs — from the Latin Anno Martyrum (in the year of the martyrs). Used in the 4th to 5th centuries, and in the Coptic calendar,
- the Anno Diocletiani, counting years from the start of the reign of Diocletian, in 284, also used in the Coptic calendar.
- The Incarnation Era is used by Ethiopia. Its epoch is August 29 8 in the Julian calendar.
- the Armenian calendar has its era fixed at AD 552.
Dionysian "Common Era"
- Main article: Anno Domini
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 to the same era. Originally intended to number years from the birth of Jesus, in fact the calculation was a few years off. Traditionally, years preceding AD 1 are numbered using the BC era (see below) to avoid zero or negative numbers.
- Note: AD was also used in the medieval Julian Calendar as well, but the calendars are not identical. To distinguish between them, 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. New Style (N.S.) was used for the Gregorian calendar.
- Note: AD is ambiguous about where in the calendar year the numbered year starts. While in modern times it is always January 1, the start of the calendar year, previously other dates were used, such as March 25. Many countries switched to using January 1 as the start of the numbered year when switching from the Julian calendar to the Gregorian calendar, but others switched earlier or later.
- B.C. (or BC) — Before Christ. Used for years prior to AD 1, counting backwards so the year n BC is the year 1-n AD. Using these two calendar eras in the intended way means that there is no year 0 or negative year numbers. However, the astronomical year numbering convention uses the AD era exclusively, so includes zero and negative year numbers.
- C.E. (or CE) — meaning Common Era, Equivalent to the Anno Domini era. This use is similar to that of Era Vulgaris (or EV) in the past. B.C.E. (or BCE) — meaning Before the Common Era. Equivalent to B.C..
Some modern calendar eras are direclty tied to the Dionysian era:
- The astronomical year numbering convention equates 1 BCE with 0 CE, and counts negative years from 2 BCE backward.
- Both the Holocene calendar proposed by Cesare Emiliani and the Jōmon era proposal set their epoch by adding 10,000 to the astronomical year number.
- The Anno Lucis of Freemasonry adds 4000 years to the AD date.
- A.H. (or AH) — for the Latinized Anno Hegirae, meaning in the year of the Hijra, Prophet Muhammad's migration from Mecca to Medina in September 622, which is taken to be the beginning of the Muslim era. This is used in the Islamic calendar. (Note that, since the Islamic calendar is a purely lunar calendar, its year count increases faster than that of solar and lunisolar calendars.)
- A.H.S. (or AHS) — used by the Iranian calendar to denote the number of solar years since the Prophet Muhammad's migration from Mecca to Medina in September 622.
- Hindu calendar, counting from the start of the Kali Yuga, with its epoch at BCE 3102 23rd January, based on Aryabhata (6th century).
- Vikrama Samvat, 56-57 BCE, introduced ca. in the 12th century.
- S.E. or (SE) — for the Saka Era, used in some Hindu calendars, epoch dated from near the vernal equinox of year 78 (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 Zoroastrian calendar used regnal years since the reform by Ardashir I, but after the fall of the Sassanid empire, the ascension of the last Sassanid ruler, Yazdegerd III of Persia, coronated June 16 632, continued to be used as reference year, abbreviated Y.Z. or "Yazdegerd era".
- A.M. (or AM) — for the Latin Anno Mundi, meaning in the year of the world, set to the year 3761 BC. This was introduced to the Hebrew calendar in ca. the 9th century, based on calculations of the date of creation by Jose ben Halafta (2nd century).
- The Republican Era of the French Republican Calendar was dated from September 22 1792, the day of the proclamation of the French First Republic.
- The Positivist calendar of 1844 is based takes 1789 as its epoch.
- The Republican era is used by the Republic of China (Taiwan) since 1929, and assigns year one to 1912, 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. Juche means "autarchy, self-reliance".
- The Italian Fascists used the standard system along with Roman numerals to denote the number of years since the establishment of the Fascist government in 1922. Therefore, 1934, for example, was Year XII. This era was abolished with the fall of fascism in Italy on July 25, 1943.
- China traditionally reckons by regnal year of emperors, see Chinese era name. Expatriate Chinese use a continuous count of years from the reign of the legendary Yellow Emperor, using the epoch 2698 BC (year 1). Western writers begin this count at either 2637 BC or 2697 BC (see Chinese calendar).
- 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.
- B.E. — for the Bahá'í Era beginning March 21 1844. See Bahá'í calendar for more details.
- 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 since founding of Bangkok on April 6, 1782. In 1912 the New Year's Day was shifted to April 1. In 1941, the Prime Minister Phibunsongkhram decided to count the years since 543 BC. This is the so-called Thai solar calendar or Thailand Buddhist Era clearly relied on the western solar calendar. This is one of the versions of the Buddhist calendar.
- B.P. — for Before Present, specifically, the number of radiocarbon years before 1950, used in radiocarbon dating.
- The Unix epoch is set at midnight of January 1, 1970.
- the Julian day number counts days, not years, and has its era fixed at January 1, 4713 BC (Julian).
- Astronomical year numbering ("Common Era" based)
- The Holocene calendar, Human Era count or Jōmon Era count (Japan) uses a similar dating system to astronomical year numbering but adds 10,000 to the year.
- Encyclopedia Britannica (2007), s.v. Jewish Era