The abbreviation AD is also widely used after the number of a century or millennium, as in 'fourth century AD' or 'second millennium AD' (although conservative usage formerly rejected such expressions). In contrast, BC is always placed after the year number (for example: AD 70, but 70 BC), which preserves syntactic order. Traditionally, English follows Latin usage by placing the 'AD' abbreviation before the year number, though it is also found after the year. This dating system was devised in 525 by Dionysius Exiguus, but was not widely used until the 9th century. There is no year zero in this scheme thus the year AD 1 immediately follows the year 1 BC. This calendar era is based on the traditionally reckoned year of the conception or birth of Jesus, AD counting years from the start of this epoch and BC denoting years before the start of the era. The form 'BC' is specific to English and equivalent abbreviations are used in other languages: the Latin form is Ante Christum natum but is rarely seen.

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'. The terms anno Domini ( AD) and before Christ ( BC) are used to label or number years in the Julian and Gregorian calendars.

Anno Domini inscription at Klagenfurt Cathedral, Austria

What does s ad mean