<P> The terms anno Domini (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendars . The term anno Domini is Medieval Latin and means "in the year of the Lord", but is often presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ". </P> <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>

What is the difference between ac and ad