Anno Domini and Before Christ (abbreviated as BC or B.C.) are designations used to number years in the Julian and Gregorian calendars. The calendar era to which they refer is based on the traditionally reckoned year of the conception or birth of Jesus, with AD denoting years after the start of this epoch, and BC denoting years before the start of this epoch. There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC.