An epoch, for the purposes of chronology and periodization, 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 (makes simple sense), 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, epoch event. In a more gradual change, a deciding moment is chosen when the epoch criterion was reached.

Epoch Examples
Anno Domini is the reference point for the Gregorian and Julian Calendars, most common in the world today
Before Present refers to 1st January 1950, used to define radio carbon dating results
The Xinhai Revolution is used as reference point for the Minguo calendar

Calendar eras

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 existed in China before 1912, being based on the accession year of the emperor (1911 was thus the third year of the Xuantong period). With the establishment of the Republic of China 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).

Pre-modern eras

Modern eras

  • The Baha'i calendar is dated from the vernal equinox of the year the Báb proclaimed his religion (AD 1844). Years are grouped in Váḥids of 19 years, and Kull-i-Shay’s of 361 (19 x 19) years.[5]
  • 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. 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 was proclaimed, one day after the Convention abolished the monarchy).
  • The Indian national calendar, introduced in 1957, follows the Saka era (AD 78).
  • Minguo calendar used by officials of Taiwan and its predecessor since January 1, 1912, the first year after Xinhai Revolution which overthrew Qing Empire and the subsequent establishment of the republic.
  • North Korea uses a system that starts in 1912 (= Juche 1), the year of the birth of their founder Kim Il-Sung.
  • In the scientific Before Present system of numbering years for purposes of radiocarbon dating, the reference date is January 1, 1950 (though the use of January 1 is quite irrelevant, as radiocarbon dating has limited precision).[6][7]
  • Different branches of Freemasonry have selected different years to date their documents according to a Masonic era, such as the Anno Lucis (A.L.).

See also


  1. ^ Blackburn, B; Holford-Strevens, L (2003). "Incarnation era". The Oxford Companion to the Year: An exploration of calendar customs and time-reckoning. Oxford University Press. p. 881.
  2. ^ Solomin, Rachel M. "Counting the Jewish Years". myjewishlearning.com.
  3. ^ Lee, Scott E. (2006). "Overview of Calendars". rosettacalendar.com.
  4. ^ Dershowitz, Nachum; Reingold, Edward M. (2008). Calendrical Calculations (3rd ed.). Cambridge University Press. p. 289. ISBN 978-0-521-70238-6.
  5. ^ Richards, E. G. (2013). "Calendars". In Urban, S. E.; Seidelman, P. K. (eds.). Explanatory Supplement to the Astronomical Almanac (3rd ed.). Mill Valley, CA: University Science Books. pp. 616–617.
  6. ^ Higham, Thomas. "Radiocarbon dating – Age calculation". c14dating.com. Thomas Higham. Retrieved December 31, 2009.
  7. ^ Stuiver, Minze; Polach HA (1977). "Discussion; reporting of C-14 data". Radiocarbon. University of Arizona. 19 (3): 355–363. Retrieved October 5, 2018.
Chronology of the universe

The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. The earliest stages of the universe's existence are estimated as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level.


The Devonian is a geologic period and system of the Paleozoic, spanning 60 million years from the end of the Silurian, 419.2 million years ago (Mya), to the beginning of the Carboniferous, 358.9 Mya. It is named after Devon, England, where rocks from this period were first studied.

The first significant adaptive radiation of life on dry land occurred during the Devonian. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods also became well-established.

Fish reached substantial diversity during this time, leading the Devonian to often be dubbed the "Age of Fishes." The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating almost every known aquatic environment. The ancestors of all four-limbed vertebrates (tetrapods) began adapting to walking on land, as their strong pectoral and pelvic fins gradually evolved into legs. In the oceans, primitive sharks became more numerous than in the Silurian and Late Ordovician.

The first ammonites, species of molluscs, appeared. Trilobites, the mollusc-like brachiopods and the great coral reefs, were still common. The Late Devonian extinction which started about 375 million years ago severely affected marine life, killing off all placodermi, and all trilobites, save for a few species of the order Proetida.

The palaeogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, and the early formation of the small continent of Euramerica in between.

Early Cretaceous

The Early Cretaceous (geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 146 Ma to 100 Ma.

During this time many new types of dinosaurs appeared or came into prominence, including ceratopsians, spinosaurids, carcharodontosaurids and coelurosaurs, while survivors from the Late Jurassic continued.

Angiosperms (flowering plants) appeared for the first time during the Early Cretaceous. This time also saw the evolution of the first members of the Neornithes (modern birds).


The Eocene ( ) Epoch, lasting from 56 to 33.9 million years ago, is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope 13C in the atmosphere was exceptionally low in comparison with the more common isotope 12C. The end is set at a major extinction event called the Grande Coupure (the "Great Break" in continuity) or the Eocene–Oligocene extinction event, which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay. As with other geologic periods, the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain.

The name Eocene comes from the Ancient Greek ἠώς (ēṓs, "dawn") and καινός (kainós, "new") and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.

Epoch (astronomy)

In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as the celestial coordinates or elliptical orbital elements of a celestial body, because these are subject to perturbations and vary with time. These time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, the node of its orbit relative to a reference plane, the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit.

The main use of astronomical quantities specified in this way is to calculate other relevant parameters of motion, in order to predict future positions and velocities. The applied tools of the disciplines of celestial mechanics or its subfield orbital mechanics (for predicting orbital paths and positions for bodies in motion under the gravitational effects of other bodies) can be used to generate an ephemeris, a table of values giving the positions and velocities of astronomical objects in the sky at a given time or times.

Astronomical quantities can be specified in any of several ways, for example, as a polynomial function of the time-interval, with an epoch as a temporal point of origin (this is a common current way of using an epoch). Alternatively, the time-varying astronomical quantity can be expressed as a constant, equal to the measure that it had at the epoch, leaving its variation over time to be specified in some other way—for example, by a table, as was common during the 17th and 18th centuries.

The word epoch was often used in a different way in older astronomical literature, e.g. during the 18th century, in connection with astronomical tables. At that time, it was customary to denote as "epochs", not the standard date and time of origin for time-varying astronomical quantities, but rather the values at that date and time of those time-varying quantities themselves. In accordance with that alternative historical usage, an expression such as 'correcting the epochs' would refer to the adjustment, usually by a small amount, of the values of the tabulated astronomical quantities applicable to a fixed standard date and time of reference (and not, as might be expected from current usage, to a change from one date and time of reference to a different date and time).

Epoch (geology)

In geochronology, an epoch is a subdivision of the geologic timescale that is longer than an age but shorter than a period. The current epoch is the Holocene Epoch of the Quaternary Period. Rock layers deposited during an epoch are called a series. Series are subdivisions of the stratigraphic column that, like epochs, are subdivisions of the geologic timescale. Like other geochronological divisions, epochs are normally separated by significant changes in the rock layers to which they correspond.

Epochs are most commonly used for the younger Cenozoic Era, where a greater collection of fossils has been found and paleontologists have more detailed knowledge of the events that occurred during those times. They are less commonly referred to for the other eras and eons, since less fossil evidence exists that allows us to form a clearer view of those time periods.

Equinox (celestial coordinates)

In astronomy, equinox is either of two places on the celestial sphere at which the ecliptic intersects the celestial equator. Although there are two intersections of the ecliptic with the celestial equator, by convention the equinox associated with the sun's ascending node is used as the origin of celestial coordinate systems and referred to simply as the equinox. In contrast to the common usage of spring and fall, or vernal and autumnal, equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.

The equinox moves because of perturbing forces, therefore in order to define a coordinate system it is necessary to specify the date for which the equinox is chosen. This date should not be confused with the epoch. Astronomical objects show real movements such as orbital and proper motions, and the epoch defines the date for which the position of an object applies. Therefore, a complete specification of the coordinates for an astronomical objects requires both the date of the equinox and of the epoch.The currently used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. The previous standard equinox and epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used. Since that time Julian equinoxes and epochs have been used.


The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years before present, after the last glacial period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene together form the Quaternary period. The Holocene has been identified with the current warm period, known as MIS 1. It is considered by some to be an interglacial period within the Pleistocene Epoch.The Holocene has seen the growth and impacts of the human species worldwide, including all its written history, development of major civilizations, and overall significant transition toward urban living in the present. Human impacts on modern-era Earth and its ecosystems may be considered of global significance for future evolution of living species, including approximately synchronous lithospheric evidence, or more recently hydrospheric and atmospheric evidence of human impacts. In July 2018, the International Union of Geological Sciences split the Holocene epoch into three distinct subsections, Greenlandian (11,700 years ago to 8,326 years ago), Northgrippian (8,326 years ago to 4,200 years ago) and Meghalayan (4,200 years ago to the present), as proposed by International Commission on Stratigraphy. The boundary stratotype of Meghalayan is a speleothem in Mawmluh cave in India, and the global auxiliary stratotype is an ice core from Mount Logan in Canada.The name Holocene comes from the Ancient Greek words ὅλος (holos, whole or entire) and καινός (kainos, new), meaning "entirely recent".

Late Cretaceous

The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series. The Cretaceous is named after the white limestone known as chalk which occurs widely in northern France and is seen in the white cliffs of south-eastern England, and which dates from this time.

Late Jurassic

The Late Jurassic is the third epoch of the Jurassic period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age. In the past, Malm was also used to indicate the unit of geological time, but this usage is now discouraged to make a clear distinction between lithostratigraphic and geochronologic/chronostratigraphic units.


The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago (Ma). The Miocene was named by Charles Lyell; its name comes from the Greek words μείων (meiōn, “less”) and καινός (kainos, “new”) and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene is preceded by the Oligocene and is followed by the Pliocene.

As the earth went from the Oligocene through the Miocene and into the Pliocene, the climate slowly cooled towards a series of ice ages. The Miocene boundaries are not marked by a single distinct global event but consist rather of regionally defined boundaries between the warmer Oligocene and the cooler Pliocene Epoch.

The Apes first evolved, arose, and diversified during the early Miocene (Aquitanian and Burdigalian stages), becoming widespread in the Old World. By the end of this epoch and the start of the following one, the ancestors of humans had split away from the ancestors of the chimpanzees to follow their own evolutionary path during the final Messinian stage (7.2 - 5.3 mya) of the Miocene. As in the Oligocene before it, grasslands continued to expand and forests to dwindle in extent. In the seas of the Miocene, kelp forests made their first appearance and soon became one of Earth's most productive ecosystems.The plants and animals of the Miocene were recognizably modern. Mammals and birds were well-established. Whales, pinnipeds, and kelp spread.

The Miocene is of particular interest to geologists and palaeoclimatologists as major phases of the geology of the Himalaya occurred during the Miocene, affecting monsoonal patterns in Asia, which were interlinked with glacial periods in the northern hemisphere.


The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present (33.9±0.1 to 23.03±0.05 Ma). As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich; the name comes from the Ancient Greek ὀλίγος (olígos, "few") and καινός (kainós, "new"), and refers to the sparsity of extant forms of molluscs. The Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period.

The Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene. Major changes during the Oligocene included a global expansion of grasslands, and a regression of tropical broad leaf forests to the equatorial belt.

The start of the Oligocene is marked by a notable extinction event called the Grande Coupure; it featured the replacement of European fauna with Asian fauna, except for the endemic rodent and marsupial families. By contrast, the Oligocene–Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the relatively cooler Miocene.


The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, and placed them in a system of their own. The Ordovician received international approval in 1960 (forty years after Lapworth's death), when it was adopted as an official period of the Paleozoic Era by the International Geological Congress.

Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events. Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event considerably increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, and those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today.


The Paleocene ( ) or Palaeocene, the "old recent", is a geological epoch that lasted from about 66 to 56 million years ago. It is the first epoch of the Paleogene Period in the modern Cenozoic Era. As with many geologic periods, the strata that define the epoch's beginning and end are well identified, but the exact ages remain uncertain.

The Paleocene Epoch is bracketed by two major events in Earth's history. It started with the mass extinction event at the end of the Cretaceous, known as the Cretaceous–Paleogene (K–Pg) boundary. This was a time marked by the demise of non-avian dinosaurs, giant marine reptiles and much other fauna and flora. The die-off of the dinosaurs left unfilled ecological niches worldwide. The Paleocene ended with the Paleocene–Eocene Thermal Maximum, a geologically brief (~0.2 million year) interval characterized by extreme changes in climate and carbon cycling.

The name "Paleocene" comes from Ancient Greek and refers to the "old(er)" (παλαιός, palaios) "new" (καινός, kainos) fauna that arose during the epoch.


The Pleistocene ( , often colloquially referred to as the Ice Age) is the geological epoch which lasted from about 2,588,000 to 11,700 years ago, spanning the world's most recent period of repeated glaciations. The end of the Pleistocene corresponds with the end of the last glacial period and also with the end of the Paleolithic age used in archaeology.

The Pleistocene is the first epoch of the Quaternary Period or sixth epoch of the Cenozoic Era. In the ICS timescale, the Pleistocene is divided into four stages or ages, the Gelasian, Calabrian, Middle Pleistocene (unofficially the 'Chibanian') and Upper Pleistocene (unofficially the 'Tarantian'). In addition to this international subdivision, various regional subdivisions are often used.

Before a change finally confirmed in 2009 by the International Union of Geological Sciences, the time boundary between the Pleistocene and the preceding Pliocene was regarded as being at 1.806 million years Before Present (BP), as opposed to the currently accepted 2.588 million years BP: publications from the preceding years may use either definition of the period.


The Pliocene ( ; also Pleiocene) Epoch is the epoch in the geologic timescale that extends from 5.333 million to 2.58 million years BP. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch. Prior to the 2009 revision of the geologic time scale, which placed the four most recent major glaciations entirely within the Pleistocene, the Pliocene also included the Gelasian stage, which lasted from 2.588 to 1.806 million years ago, and is now included in the Pleistocene.As with other older geologic periods, the geological strata that define the start and end are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The boundaries defining the Pliocene are not set at an easily identified worldwide event but rather at regional boundaries between the warmer Miocene and the relatively cooler Pliocene. The upper boundary was set at the start of the Pleistocene glaciations.


Tertiary is a widely used term for the geologic period from 66 million to 2.6 million years ago, a timespan that occurs between the Mesozoic Era and the Quaternary, although no longer recognized as a formal unit by the International Commission on Stratigraphy. The span of the Tertiary is subdivided into the Paleocene Epoch (56 – 66 million years BP), the Eocene Epoch (33.9 – 56 million years BP), the Oligocene Epoch (23 – 33.9 million years BP), the Miocene Epoch (5.3 – 23 million years BP) and the Pliocene Epoch (2.6 – 5.3 million years BP), extending to the first stage of the Pleistocene Epoch, the Gelasian stage.The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start of the Cenozoic Era, and extended to the beginning of the Quaternary glaciation at the end of the Pliocene Epoch.

The Epoch Times

The Epoch Times is a multi-language newspaper headquartered in New York City. The company was founded in 2000 by John Tang and a group of Chinese Americans associated with the Falun Gong spiritual movement. The newspaper covers general interest topics with a focus on news about China and human rights issues there. It draws from a network of sources inside China, as well as Chinese expatriates living in the West. The Epoch Times is widely distributed in overseas Chinese communities, and has been publishing in Chinese since May 2000. It is either sold or distributed free-of-charge in 35 countries, including various intranational regional editions. It has editions in English, Chinese and nine other languages in print, as well as 21 different languages on the internet. Weekly print editions are also available. A typical issue includes sections for world and national news, op-eds, sports, entertainment, business, arts and culture, travel, health and automobiles. The Epoch Times websites are blocked in mainland China, but people can access the newspaper using VPNs that function in mainland China.

Unix time

Unix time (also known as POSIX time or UNIX Epoch time) is a system for describing a point in time. It is the number of seconds that have elapsed since 00:00:00 Thursday, 1 January 1970, Coordinated Universal Time (UTC), minus leap seconds. Every day is treated as if it contains exactly 86400 seconds, so leap seconds are to be subtracted since the epoch. It is used widely in Unix-like and many other operating systems and file formats. However, Unix time is not a true representation of UTC, as a leap second in UTC shares the same Unix time as the second which came before it. Unix time may be checked on most Unix systems by typing date +%s on the command line.

On systems where Unix time is stored as a signed 32-bit number, the largest value that can be recorded is 2147483647 (231 − 1), which is 03:14:07 Tuesday, 19 January 2038 UTC. The following second, the clock will wrap around to negative 2147483648 (−231), which is 20:45:52 Friday, 13 December 1901 UTC. This is referred to as the Year 2038 problem.

Key topics
Astronomic time
Geologic time
Genetic methods
Linguistic methods
Related topics

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