What calendar, counting years from the creation of the world, was used in Byzantium?

The calendar counting years from the creation of the world used in Byzantium was the Byzantine calendar. It was an Anno Mundi system, meaning “year of the world,” based on a biblical calculation rather than on the birth of Jesus or the founding of Rome.

The Byzantine era generally placed Creation on 1 September 5509 BC, using chronology derived chiefly from the Greek Septuagint. Its calendar structure followed the Julian calendar, but the year began on 1 September instead of 1 January. Byzantine officials and chroniclers often combined this world-year system with regnal years, indictions, and other dating methods.

The system became the official calendar of the Byzantine Empire by the late 10th century and remained in use until Constantinople fell in 1453. It also continued in parts of the Orthodox Christian world, including Russia, where it was abandoned for civil use in 1700 under Peter the Great.

A common mix-up is confusing the Byzantine calendar with the Gregorian calendar. The Gregorian calendar is a later Western reform of the Julian calendar, while the Byzantine system refers especially to its Creation-era year numbering.