The proleptic Gregorian calendar is produced by extending the Gregorian calendar to dates preceding its official introduction in 1582. From March 1, 200 to February 28, 300 it corresponds to the Julian calendar. For this calendar we can distinguish two systems of numbering years BC. Bede and later historians did not use the Latin zero, nulla, as a year, so the year preceding AD 1 is 1 BC. In this system the year 1 BC is a leap year (likewise in the proleptic Julian calendar).