The Julian calendar was the calendar introduced by Julius Caesar in -45 (46 BC) to replace the Roman calendar. It has three common 365 day years, followed by a 366 day leap year in which February has 29 days. The average duration of a Julian year (365.25 days) is a poor approximation of the tropical year, which led to the Julian calendar being replaced by the Gregorian calendar. The Julian calendar is used by historians and astronomers for dates preceding its creation; it is then a fictional calendar with the same rules of construction. Historians call the year preceding year 1 of the Christian era 1 BC, a leap year; astronomers call 1 BC 0 (zero) (a leap year), 2 BC is -1 (a common year) etc.