In chronology and periodization, an epoch or reference epoch 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, an ''epoch event''. In a more gradual change, a deciding moment is chosen when the ''epoch criterion'' was reached. Calendar eras Pre-modern eras * '' Anno Mundi'' it. "Year of the World"(years since the creation of the world) is used in ** the Byzantine calendar (5509 BC). ** the Hebrew calendar (3761 BC). * The Mesoamerican Long Count Calendar uses the creation of the fourth world in 3114 BC. * Olympiads, the ancient Greek era of four-year periods between Olympic Games, beginning in 776 BC. * ''Ab urbe condita'' ("from the foundation of the city"), used to s ...