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, 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.
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).
- Olympiads, the ancient Greek era of four-year periods beginning in 776 BC.
- Ab urbe condita (753 BC), used in the Roman imperial period.
- The Anno Domini or "Common Era" era, still in use with the Gregorian and Julian calendars today, marks the Incarnation of Jesus as calculated in the 6th century by Dionysius Exiguus.
- Anno Mundi (years since the creation of the world) as used in the Byzantine calendar (5509 BC).
- Anno Mundi (years since the creation of the world) as used in the Hebrew calendar (3761 BC).
- 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 (used in Afghanistan as well as Iran) also dates from the hijra, but as it is a solar calendar its year numbering does not coincide with the religious calendar.
- The term Hindu calendar 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 "national calendar" of Nepal and Bangladesh.
- Buddhist calendars tend to use the epoch of 544 BC (date of Buddha's parinirvana).
- 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.
- 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).
- 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.
- 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).
- Different branches of Freemasonry have selected different years to date their documents according to a Masonic era, such as the Anno Lucis (A.L.).
- 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.
- Solomin, Rachel M. "Counting the Jewish Years". myjewishlearning.com.
- Lee, Scott E. (2006). "Overview of Calendars". rosettacalendar.com.
- Dershowitz, Nachum; Reingold, Edward M. (2008). Calendrical Calculations (3rd ed.). Cambridge University Press. p. 289. ISBN 978-0-521-70238-6.
- 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.
- Higham, Thomas. "Radiocarbon dating – Age calculation". c14dating.com. Thomas Higham. Retrieved December 31, 2009.
- Stuiver, Minze; Polach HA (1977). "Discussion; reporting of C-14 data". Radiocarbon. University of Arizona. 19 (3): 355–363. Retrieved October 5, 2018.