The Byzantine Empire, sometimes referred to as the Eastern Roman Empire, was the continuation of the Roman Empire in the East during Late Antiquity and the Middle Ages, when its capital city was Constantinople (modern-day Istanbul, originally founded as Byzantium). It survived the fragmentation and fall of the Western Roman Empire in the 5th century AD and continued to exist for an additional thousand years until it fell to the Ottoman Turks in 1453. During most of its existence, the empire was the most powerful economic, cultural, and military force in Europe. Both "Byzantine Empire" and "Eastern Roman Empire" are historiographical terms created after the end of the realm; its citizens continued to refer to their empire as the Roman Empire (, tr. ; ), or Romania , and to themselves as "Romans".

The Byzantine calendar, also called "Creation Era of Constantinople" or "Era of the World" (, also or ) 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.