In the year 46 BC, the Greek Sosigenes convinced Julius Caesar to reform the Roman calendar to a more manageable form. At this time, Julius also changed the number of days in the months to achieve a 365 day year. In order to "catch up" with the seasons, Julius Caesar also added 90 days to the year 46 BC between November and February (Vardi 1991, p. 238).
The Julian calendar consisted of cycles of three 365-day years followed by a 366-day leap year.Around 9 BC, it was found that the priests in charge of computing the calendar had been adding leap years every three years instead of the four decreed by Caesar (Vardi 1991, p. 239). As a result of this error, no more leap years were added until 8 AD. Leap years were therefore 45 BC, 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC, 8 AD, 12 AD, and every fourth year thereafter (Tøndering). The UNIX command cal incorrectly lists 4 AD as a leap year (Vardi 1991).