Epoch

For other uses, seeEpoch (disambiguation).

Inchronologyandperiodization,anepochorreference epochis aninstantin time chosen as the origin of a particularcalendar 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, anepoch event.In a more gradual change, adeciding momentis chosen when theepoch criterionwas reached.

Calendar eras

Pre-modern eras

• TheYoruba calendar(Kọ́jọ́dá) uses 8042 BC as the epoch, regarded as the year of the creation ofIle-Ifeby the godObatala,also regarded as the creation of the earth.
• Anno Mundi(years since the creation of the world) is used in theByzantine calendar(5509 BC).
• Anno Mundi(years since the creation of the world) as used in theHebrew calendar(3761 BC).
• TheMesoamerican Long Count Calendaruses the creation of the fourth world in 3114 BC.
• Olympiads,the ancient Greek era of four-year periods betweenOlympic Games,beginning in 776 BC.
• Ab urbe condita( "from the foundation ofthe city"), used to some extent byRoman calendarsof the Roman imperial period (753 BC).
• Buddhist calendarstend to use the epoch of 544 BC (date ofBuddha'sparinirvana).
• The termHindu calendarmay refer to a number of traditional Indian calendars. A notable example of a Hindu epoch is theVikram Samvat(58 BC),also used in modern times as the national calendars ofNepalandBangladesh.
• TheJulianandGregoriancalendars use as epoch theIncarnation of Jesusas calculated in the 6th century byDionysius Exiguus.(Subsequent research has shown that this moment is about four years after the best estimate for thedate of birth of Jesus.) This epoch was applied retrospectively to the Julian calendar, long after its original creation byJulius Caesar.
• The epoch of theIslamic calendaris theHijra(AD 622). The year count in this calendar shifts relative to the solar year count, as the calendar ispurely lunar:its year consists of 12lunationsand is thus ten or eleven days shorter than a solar year. This calendar denotes "lunar years" asAnno Hegiræ([since] the year of theHijra) orAH.This calendar is used inSunni Islamand related sects.
• The epoch of the officialIranian calendaris also theHijra,but it is asolar calendar;each year begins at the Northern spring equinox. This calendar is used inShia Islamand related sects.

Modern eras

• TheBahá'í calendaris dated from thevernal equinoxof the year theBábproclaimed his religion (AD 1844). Years are grouped inVáḥidsof 19 years, andKull-i-Shayof 361 (19×19) years.
• InThailandin 1888 KingChulalongkorndecreed a National Thai Era dating from the founding ofBangkokon April 6, 1782. In 1912, New Year's Day was shifted to April 1. In 1941,Prime MinisterPhibunsongkhramdecided to count the years since 543 BC. This is theThai solar calendarusing the Thai Buddhist Era. Except for this era, it is the Gregorian calendar.
• In theFrench 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 theFrench First Republicwas proclaimed, one day after the Convention abolished theAncien Regime).
• TheIndian national calendar,introduced in 1957, follows theSaka era(AD 78).
• TheMinguo calendarused by officials ofTaiwanandits predecessordates from January 1, 1912, the first year after theXinhai Revolution,which overthrew theQing Empire.
• North Koreauses a system that starts in 1912 (=Juche1), the year of the birth of its founderKim Il-Sung.
• TheFascist Eradates toMussolini'sMarch on Romein 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 theItalian Social Republicin 1945.
• In the scientificBefore Presentsystem of numbering years for purposes ofradiocarbon dating,the reference date is January 1, 1950 (though the specific date January 1 is quite unnecessary, as radiocarbon dating has limited precision).
• Different branches ofFreemasonryhave selected different years to date their documents according to a Masonic era, such as theAnno Lucis(A.L.).
• TheHolocene calendaruses 10,000 BC as the epoch, the beginning of theHolocene epochon thegeological time scale.

Regnal eras

Further information:Regnal era

The officialJapanese systemnumbers years from the accession of the currentemperor,regarding the calendar year during which the accession occurred as the first year. A similarsystemexisted inChinabefore 1912, being based on the accession year of the emperor (1911 was thus the third year of theXuantongperiod). With the establishment of theRepublic of Chinain 1912, the republican era was introduced. It is still very common inTaiwanto 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

Anepoch in computingis the time at which the representation is zero. For example,Unix timeis represented as the number of seconds since 00:00:00 UTC on 1 January 1970, not countingleap seconds.

Anepoch in astronomyis 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.

An epoch inGeochronologyis a period of time, typically in the order of tens of millions of years. The current epoch is theHolocene.

See also

• Dating creation– Using creation myths to date the Earth
• Era– Span of time defined for the purposes of chronology or historiography
• Geologic time scale– System that relates geologic strata to time
• Lunisolar calendar– Calendar with lunar month, solar year
• Metonic cycle– 19-year pattern in lunisolar calendars
• Saros (astronomy)– Cycles used to predict eclipses of the Sun and Moon
• Timekeeping on Mars– Proposed approaches to tracking date and time on the planet Mars