Epoch (reference date)

In the fields of chronology and periodization, an epoch is an instant in time chosen as the origin of a particular era. The "epoch" then serves as a reference point from which time is measured. Time measurement units are counted from the epoch so that the date and time of events can be specified unambiguously.

Events taking place before the epoch can be dated by counting negatively from the epoch, though in pragmatic periodization practice, epochs are defined for the past, and another epoch is used to start the next era, therefore serving as the ending of the older preceding era. The whole purpose and criteria of such definitions are to clarify and co-ordinate scholarship about a period, at times, across disciplines.

Epochs are generally chosen to be convenient or significant by a consensus of the time scale's initial users, or by authoritarian fiat. The epoch moment or date is usually defined by a specific clear event, condition, or criterion—the epoch event or epoch criterion—from which the period or era or age is usually characterized or described.

Epoch Examples
by events
The assassination of the Roman Emperor Alexander Severus triggering the Crisis of the Third Century
The defenestration of Prague triggering the Thirty Years' War
Queen Victoria ascending to the throne giving the start of the Victorian era
by criteria
The spurt in exploration, mercantilism, and colonization in the Age of Discovery
Particular ratios of animal fossils in a rock strata —various Geology epochs

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.).
Other Languages
bosanski: Epoha
Esperanto: Epoko
français: Epoch
magyar: Epocha
日本語: 紀元
norsk nynorsk: Epoke
sicilianu: Èbbica
کوردی: ئیپۆک
српски / srpski: Епоха (датум)
srpskohrvatski / српскохрватски: Epoha (datum)
svenska: Epok
українська: Епоха