Julian Calendar took effect on 1 January 709 AUC (45 BC)
The Julian calendar, proposed by Julius Caesar in 708 AUC (46 BC), was a reform of the Roman calendar. It took effect on 1 January 709 AUC (45 BC), by edict. It was designed with the aid of Greek mathematicians and Greek astronomers such as Sosigenes of Alexandria.
The calendar was the predominant calendar in the Roman world, most of Europe, and in European settlements in the Americas and elsewhere, until it was gradually replaced by the Gregorian calendar, promulgated in 1582 by Pope Gregory XIII. The Julian calendar is still used in parts of the Eastern Orthodox Church, in parts of Oriental Orthodoxy and Anabaptism, as well as by the Berbers.
During the 20th and 21st centuries, a date according to the Julian calendar is 13 days earlier than (behind) its corresponding Gregorian date.