Byzantine calendar

The Byzantine calendar, also called "Creation Era of Constantinople" or "Era of the World" (Ancient Greek: Ἔτη Γενέσεως Κόσμου κατὰ Ῥωμαίους, also Ἔτος Κτίσεως Κόσμου or Ἔτος Κόσμου), was the calendar used by the Eastern Orthodox Church from c. 691 to 1728 in the Ecumenical Patriarchate. It was also the official calendar of the Byzantine Empire from 988 to 1453, and of Kievan Rus' and Russia from c. 988 to 1700.

The calendar was based on the Julian calendar, except that the year started on 1 September and the year number used an Anno Mundi epoch derived from the Septuagint version of the Bible. It placed the date of creation at 5509 years before the Incarnation, and was characterized by a certain tendency which had already been a tradition among Jews and early Christians to number the years from the foundation of the world (Latin: Annus Mundi or Ab Origine Mundi— "AM"). Its year one, the supposed date of creation, was September 1, 5509 BC, to August 31, 5508 BC.

It is not known who invented the World era and when. Nevertheless, the first appearance of the term is in the treatise of a certain "monk and priest", Georgios (AD 638–39), who mentions all the main variants of the "World Era" in his work. Georgios argues that the main advantage of the World era is the common starting point of the astronomical lunar and solar cycles, and of the cycle of indictions, the usual dating system in Byzantium since the 6th century. He also already regards it as the most convenient for the Easter computus. Complex calculations of the 19-year lunar and 28-year solar cycles within this world era allowed scholars to discover the cosmic significance of certain historical dates, such as the birth of Christ or the Crucifixion.

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