Byzantine calendar also called Era of the World was the calendar used on the Eastern Ortodox Church from 691 to 1728. The calendar was based on the Julian calendar but the year started on 1 September and the year number used an Anno Mundi epoch derived from Septuagint version of the Bible.
It is not who invented this era and when, however it appears for a "monkand priest" Georgios (AD 638-39) who mentions all the main variants of the calendar in his work. Georgios makes it clear that the main advantage of the Byzantine era is the starting point of the astronomical lunar and solar cycles and of the cycle of indications, the usual dating system in Bizantium since sixth century.
The Era was calculated starting on September 1st, and Jesus was born in the year 5509 Anno Mundi- the year since the creation of the world. That historical time was calculated frm the creation and not frm the birth of Jesus, as in the west. However the Byzantine Calendar was identical to the Julian calendar excepted that:
- names of the month were transcribed form the latin into greek.
- the first day of the year was September 1st.
- the date of creation was September 1st, 5509 BC to August 31st 5508 BC
Earliest christian sources of the Age of the World
The earliest Christian sources on the Age of the world according to biblical chronology are by St. Theophilus of Antioch (AD 115-181) the sixth bishop of Antioch from the Apostles, in his apologetic work To Autolycus, and by Julius Africanus (AD 200-245) in his Five Books of Chronology. Both of these early Christian writers, following the Septuagint version of the Old Testament, determined the age of the world to have been about 5,530 years at the birth of Christ.
The "Alexandrian Era" (Greek: Κόσμου ἔτη κατ’ Ἀλεξανδρεῖς ) developed in AD 412, was the precursor to the Byzantine Era. After the initial attempts by Hippolytus, Clement of Alexandria and others, the Alexandrian computation of the date of creation was worked out to be March 25, 5493 BC.
The Alexandrine monk Panodoros reckoned 5904 years from Adam to the year AD 412. His years began with August 29, corresponding to the First of Thoth, or the Egyptian new year. Bishop Annianos of Alexandria, however, preferred the Annunciation style as New Year´s Day, the 25th of March, and shifted the Panodoros era by about six months, to begin on March 25. This created the Alexandrian Era, whose first day was the first day of the proleptic Alexandrian civil year in progress, August 29, 5493 BC, with the ecclesiastical year beginning on March 25, 5493 BC.