Calendar era

A system for numbering years from a chosen reference event or epoch. Its year numbers locate dates relative to that starting point.

Epoch: A fixed point in time used as the origin for measuring dates or elapsed time. An era’s reference event establishes the epoch from which its years are numbered.
Anno Domini: The year-numbering convention that counts years from the traditionally calculated birth of Jesus. It is the dominant example of an era whose reference point is a religious figure.
Regnal year: A year counted from the beginning of a monarch’s reign. It numbers years from successive reigns rather than from one enduring reference event.
Dionysius Exiguus: A sixth-century monk who devised a Christian year count later called Anno Domini. His Easter tables helped introduce the year numbering that became the common Western era.
Calendar: A system for organizing time into units such as days, months, and years. An era supplies year numbers within a calendar’s broader structure.
Hijri year: A year in the Islamic calendar, numbered from the migration of Muhammad from Mecca to Medina. Its epoch is the Hijra, while its lunar calendar makes its years shorter than solar years.
Julian day: A continuous count of days elapsed from a fixed starting point in 4713 BCE. It counts days continuously, unlike an era’s calendar-based numbering of years.
Bede: An eighth-century English monk and historian who helped popularize Anno Domini dating. His historical writings spread the Christian era beyond its earlier computational use.
Year numbering: The assignment of numbers to successive years in a chronological system. Calendar eras differ chiefly in the starting point and conventions used for numbering years.
Buddhist calendar: A family of calendars that number years from the traditional death of the Buddha. It illustrates an era anchored to a religious figure’s traditionally dated death.

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