Anno Domini (abbreviated as AD or A.D.) and Before Christ (abbreviated as BC or B.C.) are designations used to label or number years used with the Julian and Gregorian calendars. This calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, with AD counting years after the start of this epoch, and BC denoting years before the start of the epoch. Alternatively, the secular abbreviations CE and BCE are used, respectively. There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC. This dating system was devised in 525, but was not widely used until after 800.