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In
chronology Chronology (from Latin ''chronologia'', from Ancient Greek , ''chrónos'', "time"; and , ''wikt:-logia, -logia'') is the science of arranging events in their order of occurrence in time. Consider, for example, the use of a timeline or sequence ...
and
periodization Periodization is the process or study of categorizing the past into discrete, quantified named blocks of time.Adam Rabinowitz. It’s about time: historical periodization and Linked Ancient World Data'. Institute for the Study of the Ancient Wo ...
, an epoch or reference epoch is an
instant In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "P ...

instant
in time chosen as the origin of a particular
calendar era #REDIRECT Calendar era #REDIRECT Calendar era A calendar era is the period of time elapsed since one ''epoch'' of a calendar and, if it exists, before the next one. For example, it is the year as per the Gregorian calendar, which numbers its year ...
. 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


Regnal eras

The official Japanese system numbers years from the accession of the current
emperor An emperor (from la, imperator The Latin word "imperator" derives from the stem of the verb la, imperare, label=none, meaning 'to order, to command'. It was originally employed as a title roughly equivalent to ''commander'' under the Roma ...
, 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, is described by its boundaries, structure and purp ...
existed in
China China (), officially the People's Republic of China (PRC; ), is a country in East Asia East Asia is the eastern region of Asia Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere ...

China
before 1912, being based on the accession year of the emperor (1911 was thus the third year of the
Xuantong Puyi (; February 7, 1906 – October 17, 1967), courtesy name Yaozhi (曜之), was the last emperor of China as the eleventh and final Qing dynasty emperor, Qing dynasty ruler. He became the Xuantong Emperor (then spelled as Hsuan Tung Emper ...

Xuantong
period). With the establishment of the
Republic of China Taiwan, officially the Republic of China (ROC), is a country in East Asia. It shares Maritime boundary, maritime borders with the China, People's Republic of China (PRC) to the northwest, Japan to the northeast, and the Philippines to the sout ...
in 1912, the republican era was introduced. It is still very common in
Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia East Asia is the eastern region of Asia Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere, Eastern and N ...

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).


Pre-modern eras

*
Olympiad An Olympiad ( el, Ὀλυμπιάς, ''Olympiás'') is a period of four years associated with the Olympic Games The modern Olympic Games or Olympics (french: Jeux olympiques) are leading international sporting events featuring summer and ...

Olympiad
s, the ancient Greek era of four-year periods between
Olympic Games The modern Olympic Games or Olympics (french: Jeux olympiques) are leading international sporting events featuring summer and winter sports competitions in which thousands of athletes An athlete (also sportsman or sportswoman) is a pe ...
, beginning in 776 BC. * ''
Ab urbe condita 300px, Antoninianus of Pacatianus, Roman usurper, usurper of Roman emperor Philip the Arab, Philip in 248. It reads ''ROMAE AETERANMIL ESIMOET PRIMO'', 'To eternal Rome, in its one thousand and first year.' ''Ab urbe condita'' ( ...
'' (753 BC), used in the Roman imperial period. * The ''
Anno Domini The terms (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendar The Gregorian calendar is the used in most of the world. It was introduced in October 1582 by as a modification of the , r ...
'' or "
Common Era Common Era (CE) is one of the year notations used for the Gregorian calendar (and its predecessor, the Julian calendar), the world's most widely used calendar era. Before the Common Era (BCE) is the era before CE. BCE and CE are alternatives ...
" system, still in use with the Gregorian and
Julian calendar The Julian calendar, proposed by Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century B ...
s today, marks the Incarnation of Jesus as calculated in the 6th century by
Dionysius Exiguus Dionysius Exiguus (Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republ ...

Dionysius Exiguus
. * '' Anno Mundi'' (years since the creation of the world) is used in the
Byzantine calendar The Byzantine calendar, also called "Creation Era of Constantinople" or "Era of the World" ( grc, Ἔτη Γενέσεως Κόσμου κατὰ Ῥωμαίους, also or , abbreviated as ε.Κ.; literal translation of ancient Greek "Roman ye ...
(5509 BC). * '' Anno Mundi'' (years since the creation of the world) as used in the
Hebrew calendar The Hebrew calendar ( Hebrew: , ), also called Jewish calendar, is a lunisolar calendar used today for Jewish religious observance, and as an official calendar of the state of Israel Israel (; he, יִשְׂרָאֵל; ar, إِس ...
(3761 BC). * The
Islamic calendar The Hijri calendar ( ar, ٱلتَّقْوِيم ٱلْهِجْرِيّ '), also known as the Lunar Hijri calendar and (in English) as the Islamic, Muslim or Arabic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 o ...
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 The Iranian calendars or Iranian chronology ( fa, گاه‌شماری ایرانی, ''Gāh-Šomāri-ye Irāni'') are a succession of calendars invented or used for over two millennia in , also known as Persia. One of the longest chronological re ...
(also used in
Afghanistan Afghanistan (; Pashto Pashto (,; / , ), sometimes spelled Pukhto or Pakhto, is an Eastern Iranian language The Eastern Iranian languages are a subgroup of the Iranian languages The Iranian or Iranic languages are a branch of t ...

Afghanistan
) dates from the ''Hijra'', but as it is a
solar calendar A solar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day within ...
, its year numbering does not coincide with the religious calendar. * The term
Hindu calendar The Hindu calendar, Panchanga (Sanskrit Sanskrit (, attributively , ''saṃskṛta-'', nominalization, nominally , ''saṃskṛtam'') is a classical language of South Asia belonging to the Indo-Aryan languages, Indo-Aryan branch of the Indo- ...

Hindu calendar
may refer to a number of traditional Indian calendars. A notable example of a Hindu epoch is the ''
Vikram Samvat Vikram Samvat (IAST: ''Vikrama Samvat''; abbreviated VS) or Bikram Sambat (abbreviated as BS) and also known as the Vikrami calendar, is the historical Hindu calendar used in the Indian subcontinent. It is the official calendar of Nepal. In India ...
'' (58 BC), also used in modern times as the national calendars of
Nepal Nepal (; ne, नेपाल ), officially the Federal Democratic Republic of Nepal ( ne, सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is ma ...

Nepal
and
Bangladesh Bangladesh (, bn, বাংলাদেশ, ), officially the People's Republic of Bangladesh, is a country in South Asia South Asia is the southern region of Asia, which is defined in both geography, geographical and culture, ethno-c ...

Bangladesh
. *
Buddhist calendar The Buddhist calendar is a set of lunisolar calendar A lunisolar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calenda ...
s tend to use the epoch of 544 BC (date of
Buddha Gautama Buddha, popularly known as the Buddha (also known as Siddhattha Gotama or Siddhārtha Gautama or Buddha Shakyamuni), was an ascetic Asceticism (; from the el, ἄσκησις ''áskesis'', "exercise, training") is a lifestyle ...

Buddha
's ''
parinirvana , or Mahaparinirvana, Gandhara Gandhāra ( sa, गन्धार, link=no) was an ancient region in the Valley of Peshawar, Peshawar basin in the far north-west of the ancient Indian subcontinent, corresponding to present-day north-west Pak ...

parinirvana
''). * The Yoruba calendar (''Kọ́jọ́dá'') uses 8042 B.C. 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.


Modern eras

* The Bahá'í calendar is dated from the vernal equinox of the year the
Báb The Báb, born Sayyed `Alí Muḥammad Shírází (; fa, سيد علی ‌محمد شیرازی; October 20, 1819 – July 9, 1850) was the founder of Bábism The Bábi Faith ( fa, بابیه, ''Babiyye'') is an Abrahamic religions, Abrah ...
proclaimed his religion (AD 1844). Years are grouped in ''Váḥids'' of 19 years, and ''Kull-i-Shay'' of 361 (19×19) years. * In
Thailand Thailand ( th, ประเทศไทย), historically known as Siam, () officially the Kingdom of Thailand, is a country in Southeast Asia. It is located at the centre of the Mainland Southeast Asia, Indochinese Peninsula, spanning , wi ...

Thailand
in 1888 King
Chulalongkorn Chulalongkorn ( th, จุฬาลงกรณ์, 20 September 1853 – 23 October 1910) was the fifth monarch A monarch is a head of state A head of state (or chief of state) is the public persona A persona (plural personae or ...

Chulalongkorn
decreed a National Thai Era dating from the founding of
Bangkok Bangkok is the capital and most populous city of Thailand Thailand ( th, ประเทศไทย), historically known as Siam, () officially the Kingdom of Thailand, is a country in Southeast Asia Southeast Asia or Southeastern ...

Bangkok
on April 6, 1782. In 1912, New Year's Day was shifted to April 1. In 1941,
Prime Minister A prime minister or a premier is the head of the cabinet Cabinet or The Cabinet may refer to: Furniture * Cabinetry, a box-shaped piece of furniture with doors and/or drawers * Display cabinet, a piece of furniture with one or more transpa ...
Phibunsongkhram decided to count the years since 543 BC. This is the
Thai solar calendar The Thai solar calendar ( th, ปฏิทินสุริยคติ, , "solar calendar") was adopted by King Chulalongkorn Chulalongkorn ( th, จุฬาลงกรณ์, 20 September 1853 – 23 October 1910) was the fifth monarc ...
using the Thai Buddhist Era. Except for this era, it is the Gregorian calendar. * In the
French Republican Calendar The French Republican calendar (french: calendrier républicain français), also commonly called the French Revolutionary calendar (), was a calendar A calendar is a system of organizing days. This is done by giving names to periods of ...
, 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''), officially the French Republic (''République française''), was founded on 21 September 1792 during the French Revolution. The First Republic lasted until the dec ...
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 {{disambig ...
''). * The
Indian national calendar The Indian national calendar, sometimes called the Saka calendar, is used, alongside the Gregorian calendar, by ''The Gazette of India'', in news broadcasts by All India Radio and in calendars and communications issued by the Government of India. ...

Indian national calendar
, introduced in 1957, follows the
Saka era 250px, Mohar of Gorkhali king Prithvi Narayan Shah dated Shaka era 1685 (AD 1763) The Shaka era (IAST The International Alphabet of Sanskrit Transliteration (IAST) is a transliteration scheme that allows the lossless romanisation of Brahmic ...
(AD 78). * The
Minguo calendar The Republic of China calendar () or Minguo calendar () is one of the calendars used in the Greater China "Greater China" is an informal geographical area that shares commercial and cultural ties with the Han Chinese The Han Chi ...
used by officials of
Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia East Asia is the eastern region of Asia Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere, Eastern and N ...

Taiwan
and its predecessor dates from January 1, 1912, the first year after the
Xinhai Revolution The 1911 Revolution, also known as the Chinese Revolution or the Xinhai Revolution, ended China's last imperial dynasty, the Manchu The Manchu (; ) are an officially recognized ethnic minority in China and the people from whom Manchuria ...
, which overthrew the
Qing Empire The Qing dynasty, officially the Great Qing (), was the last dynasty A dynasty (, ) is a sequence of rulers from the same family,''Oxford English Dictionary'', "dynasty, ''n''." Oxford University Press Oxford University ...
. *
North Korea North Korea, officially the Democratic People's Republic of Korea (DPRK), is a country in East Asia, constituting the northern part of the Korean Peninsula. It borders China and Russia to the north, at the Yalu River, Yalu (Amnok) and Tu ...

North Korea
uses a system that starts in 1912 (=
Juche ''Juche'' ( ), officially the ''Juche'' idea and also translated as ''Juche'' thought and Jucheism, is the state ideology of North Korea, described by the country's government as "Kim Il-sung's original, brilliant and revolutionary contribution ...

Juche
1), the year of the birth of its founder
Kim Il-Sung , relatives = Kim family , residence = Pyongyang, North Korea , profession = Politician , allegiance = , branch = Red Army The Workers' and Peasants' Red Army,) frequently short ...

Kim Il-Sung
. * The
Fascist Era Italian Fascism ( it, fascismo italiano), also known as Classical Fascism or simply Fascism, is the original Fascism, fascist ideology as developed in Italy by Giovanni Gentile and Benito Mussolini. The ideology is associated with a series of tw ...
dates to
Mussolini
Mussolini
's
March on Rome The March on Rome ( it, Marcia su Roma) was an organized mass demonstration in October 1922 which resulted in Benito Mussolini's National Fascist Party (PNF) ascending to power in the Kingdom of Italy The Kingdom of Italy ( it, Regno d'Itali ...

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), popularly and historically known as the Republic of Salò ( it, Repubblica di Salò ), was a List of World War II puppet states#Germany, German puppet state with limited D ...

Italian Social Republic
in 1945. * In the scientific
Before Present Before Present (BP) years is a time scale used mainly in archaeology Archaeology or archeology is the study of human activity through the recovery and analysis of material culture. Archaeology is often considered a branch of socio-cultural ...
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 Organic matter, organic material, or natural organic matter refers to the large source of ...
, 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 Freemasonry or Masonry refers to fraternal organisations that trace their origins to the local guilds of stonemasons Stonemasonry or stonecraft is the creation of building A building, or edifice, is a structure with a roof and walls ...

Freemasonry
have selected different years to date their documents according to a Masonic era, such as the '' Anno Lucis'' (A.L.). * The
Holocene calendar The Holocene calendar, also known as the Holocene Era or Human Era (HE), is a year numbering system that adds exactly 10,000 years to the currently dominant ( AD/ BC or CE/BCE Common Era (CE) is one of the year notations used for the Gre ...
uses 10,000 BC as the epoch, the beginning of the
Holocene epoch The Holocene ( ) is the current geological epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language belongi ...
on the
geological time scale The geologic time scale (GTS) is a system of chronological dating that classifies geological strata (stratigraphy through Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that ...
.


Other applications

An epoch in computing is the time at which the representation is zero. For example,
Unix time Current Unix time () Unix time (also known as Epoch time, POSIX time, seconds since the Epoch, or UNIX Epoch time) is a system for describing a point in time Time is the indefinite continued progress of existence and events that occur in an ...
is represented as the number of seconds since 00:00:00 UTC on 1 January 1970, not counting
leap second A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar tim ...

leap second
s. 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 The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) exists to promote and safeguard t ...
.


See also

* Dating creation *
Geologic time scale The geologic time scale (GTS) is a system of chronological dating Chronological dating, or simply dating, is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously establi ...

Geologic time scale
*
Lunisolar calendar A lunisolar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day ...
*
Metonic cycle For example, by the 19-year Metonic cycle, the full moon repeats on or near Christmas day between 1711 and 2300. A small horizontal libration is visible comparing their appearances. A red color shows full moons that are also lunar eclipses. The ...

Metonic cycle
*
Saros (astronomy)The saros () is a period of exactly 223 synodic months, approximately 6585.3211 days, or 18 years, 10, 11, or 12 days (depending on the number of leap years), and 8 hours, that can be used to predict eclipses of the Sun and Moon The Moon ...
*
Timekeeping on Mars Though no standard exists, numerous calendars and other timekeeping approaches have been proposed for the planet Mars. The most commonly seen in scientific literature denotes the time of year as the number of degrees from the northern vernal equin ...
*


References

{{Authority control Calendar eras Calendaring standards Chronology