Epochs of Roman Emperors
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In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by congruity, or by following conventions understood from the epoch in question. The epoch moment or date is usually defined from a specific, clear event of change, an ''epoch event''. In a more gradual change, a deciding moment is chosen when the ''epoch criterion'' was reached.


Calendar eras


Pre-modern eras

* The
Yoruba calendar The Yoruba calendar (''Kọ́jọ́dá'') is a calendar used by the Yoruba people of southwestern and north central Nigeria and southern Benin. The calendar has a year beginning on the last moon of May or first moon of June of the Gregorian cal ...
(''Kọ́jọ́dá'') uses 8042 BC as the epoch, regarded as the year of the creation of Ile-Ife by the god Obatala, also regarded as the creation of the earth. * '' Anno Mundi'' (years since the creation of the world) is used in the
Byzantine calendar The Byzantine calendar, also called the Roman calendar, the Creation Era of Constantinople or the Era of the World ( grc, Ἔτη Γενέσεως Κόσμου κατὰ Ῥωμαίους, also or , abbreviated as ε.Κ.; literal translation of ...
(5509 BC). * '' Anno Mundi'' (years since the creation of the world) as used in the Hebrew calendar (3761 BC). * Olympiads, the ancient Greek era of four-year periods between Olympic Games, beginning in 776 BC. * ''
Ab urbe condita ''Ab urbe condita'' ( 'from the founding of the City'), or ''anno urbis conditae'' (; 'in the year since the city's founding'), abbreviated as AUC or AVC, expresses a date in years since 753 BC, the traditional founding of Rome. It is an exp ...
'' (753 BC), used in the Roman imperial period. * Buddhist calendars tend to use the epoch of 544 BC (date of Buddha's '' parinirvana''). * The term
Hindu calendar The Hindu calendar, Panchanga () or Panjika is one of various lunisolar calendars that are traditionally used in the Indian subcontinent and Southeast Asia, with further regional variations for social and Hindu religious purposes. They adopt a s ...
may refer to a number of traditional Indian calendars. A notable example of a Hindu epoch is the '' Vikram Samvat'' (58 BC), also used in modern times as the national calendars of Nepal and Bangladesh. * The '' Anno Domini'' or '' Common Era'' system, still in use with the Julian calendar and Gregorians today, marks the
Incarnation of Jesus In Christian theology, the incarnation is the belief that the pre-existent divine person of Jesus Christ, God the Son, the second person of the Trinity, and the eternally begotten ''Logos'' (Koine Greek for "word"), took upon human nature and ...
as calculated in the 6th century by Dionysius Exiguus. * The Islamic calendar counts "lunar years" by ''Anno Hegiræ'' (in the year of the '' Hijra'') or ''AH'' era (AD 622). The year count shifts relative to the solar year as the calendar is purely lunar. The official Iranian calendar (also used in Afghanistan) dates from the ''Hijra'', but as it is a
solar calendar A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar. T ...
, its year numbering does not coincide with the religious calendar.


Modern eras

* The Bahá'í calendar is dated from the
vernal equinox Spring equinox or vernal equinox or variations may refer to: * March equinox, the spring equinox in the Northern Hemisphere * September equinox, the spring equinox in the Southern Hemisphere Other uses * Nowruz, Persian/Iranian new year which be ...
of the year the Báb proclaimed his religion (AD 1844). Years are grouped in ''Váḥids'' of 19 years, and ''Kull-i-Shay'' of 361 (19×19) years. * In Thailand in 1888 King
Chulalongkorn Chulalongkorn ( th, จุฬาลงกรณ์, 20 September 1853 – 23 October 1910) was the fifth monarch of Siam under the House of Chakri, titled Rama V. He was known to the Siamese of his time as ''Phra Phuttha Chao Luang'' (พร ...
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 Field Marshal Plaek Phibunsongkhram ( th, แปลก พิบูลสงคราม ; alternatively transcribed as ''Pibulsongkram'' or ''Pibulsonggram''; 14 July 1897 – 11 June 1964), locally known as Marshal P. ( th, จอมพล ...
decided to count the years since 543 BC. This is the Thai solar calendar using the Thai Buddhist Era. Except for this era, it is the Gregorian calendar. * In the French Republican Calendar, a calendar used by the French government for about twelve years from late 1793, the epoch was the beginning of the "Republican Era", September 22, 1792 (the day the
French First Republic In the history of France, the First Republic (french: Première République), sometimes referred to in historiography as Revolutionary France, and officially the French Republic (french: République française), was founded on 21 September 1792 ...
was proclaimed, one day after the Convention abolished the ''
Ancien Regime ''Ancien'' may refer to * the French word for "ancient, old" ** Société des anciens textes français * the French for "former, senior" ** Virelai ancien ** Ancien Régime ** Ancien Régime in France ''Ancien'' may refer to * the French word for ...
''). * The Indian national calendar, introduced in 1957, follows the Saka era (AD 78). * The Minguo calendar used by officials of Taiwan and its predecessor dates from January 1, 1912, the first year after the Xinhai Revolution, which overthrew the
Qing Empire The Qing dynasty ( ), officially the Great Qing,, was a Manchu-led imperial dynasty of China and the last orthodox dynasty in Chinese history. It emerged from the Later Jin dynasty founded by the Jianzhou Jurchens, a Tungusic-speak ...
. * North Korea uses a system that starts in 1912 (= Juche 1), the year of the birth of its founder Kim Il-Sung. * The Fascist Era dates to Mussolini's March on Rome in 1922, and was in use only in countries under hegemony of the Fascist regime of Benito Mussolini. It has been defunct since the fall of the
Italian Social Republic The Italian Social Republic ( it, Repubblica Sociale Italiana, ; RSI), known as the National Republican State of Italy ( it, Stato Nazionale Repubblicano d'Italia, SNRI) prior to December 1943 but more popularly known as the Republic of Salò ...
in 1945. * In the scientific Before Present system of numbering years for purposes of
radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was dev ...
, the reference date is January 1, 1950 (though the specific date January 1 is quite unnecessary, as radiocarbon dating has limited precision). * Different branches of Freemasonry have selected different years to date their documents according to a Masonic era, such as the ''
Anno Lucis Anno Lucis (“in the Year of Light”) is a dating system used in Masonic ceremonial or commemorative proceedings, which is equivalent to the Gregorian year plus 4000. It is similar to ''Anno Mundi''. Description For example, a date Anno Domini ...
'' (A.L.). * The Holocene calendar uses 10,000 BC as the epoch, the beginning of the Holocene epoch on the
geological time scale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronol ...
.


Regnal eras

The official Japanese system numbers years from the accession of the current emperor, regarding the calendar year during which the accession occurred as the first year. A similar
system A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment (systems), environment, is described by its boundaries, ...
existed in
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
before 1912, being based on the accession year of the emperor (1911 was thus the third year of the
Xuantong Aisin-Gioro Puyi (; 7 February 1906 – 17 October 1967), courtesy name Yaozhi (曜之), was the last emperor of China as the eleventh and final Qing dynasty monarch. He became emperor at the age of two in 1908, but was forced to abdicate on 1 ...
period). With the establishment of the
Republic of China Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the northeast ...
in 1912, the republican era was introduced. It is still very common in Taiwan to date events via the republican era. The People's Republic of China adopted the common era calendar in 1949 (the 38th year of the Chinese Republic).


Other applications

An epoch in computing is the time at which the representation is zero. For example, Unix time is represented as the number of seconds since 00:00:00 UTC on 1 January 1970, not counting leap seconds. An epoch in astronomy is a reference time used for consistency in calculation of positions and orbits. A common astronomical epoch is J2000, which is noon on January 1, 2000,
Terrestrial Time Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth. For example, the Astronomical Almanac uses T ...
. An epoch in Geochronology is a period of time, typically in the order of tens of millions of years. The current epoch is the Holocene.


See also

* * * * * * *


References

{{Authority control Calendar eras Calendaring standards Chronology