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Dionysius Exiguus invented Anno Domini years to date Easter.
Anno Domini (abbreviated as AD or A.D., sometimes found in the irregular form Anno Domine) and Before Christ (abbreviated as BC or B.C.) are designations used to label 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. This dating system was devised in AD 525, but was not widely used until after AD 800.[1]
The Gregorian calendar, and the year numbering system associated with it, is the calendar system with the most widespread use in the world today. For decades, it has been the unofficial global standard, recognized by international institutions such as the United Nations and the Universal Postal Union. It is also a basis of scholarly dating, though some people adopt the Common/Christian Era labels, retaining the same numeric values but using the label "CE" (Common/Christian Era) instead of "AD", and "BCE" (Before the Common/Christian Era) instead of "BC".
The term Anno Domini is Medieval Latin, translated as In the year of (the/Our) Lord.[2][3]:782 It is sometimes specified more fully as Anno Domini Nostri Iesu (Jesu) Christi ("In the Year of Our Lord Jesus Christ").
Traditionally, English has copied Latin usage by placing the abbreviation before the year number for AD; since BC is not derived from Latin it is placed after the year number (for example: 68 BC, but AD 2010). However, placing the AD after the year number (as in "2010 AD") is now also common. The abbreviation 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