<P> This calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth , with AD counting years from the start of this epoch , and BC denoting years before the start of the era . 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 by Dionysius Exiguus of Scythia Minor , but was not widely used until after 800 . </P>
when did it change from bc to ad
year of the conception or birth of Jesus of Nazareth