AD (Anno Domini)

AD — from the Latin Anno Domini, meaning "in the year of the Lord" — is a notation for numbering years in the Julian and Gregorian calendars. It designates years after a reference point from which counting begins, based on a calculation of the birth of Jesus (Christ). Years before this reference point are marked BC (before Christ), from the Latin "before the birth of Christ".

This system is now the most widely used in official documents, commerce, and computer calculations worldwide. Its use is not purely religious: international organizations and different states that follow other faiths use it for the practical reason that a single shared system facilitates communication.

Origins and adoption

The system is linked to a method of calculating time developed by a monk and scholar in the early centuries, who sought to establish a way to reckon the date of the Christian Easter without reference to the Roman emperors. It was used initially only in church documents. Centuries later it spread to European kingdoms and, following trade and colonial administration, to much of the world.

How years are counted

This system has no year zero: the year 1 BC is immediately followed by the year 1 AD. Therefore, when calculating the span between two dates on either side of the reference point, one must subtract or add one year. Astronomers use a different system that includes a year zero to simplify calculations.

Alternative terms in use

Scientific history texts and some international publications use secular notation: CE (Common Era) and BCE (Before Common Era). The numbering and years are identical; only the labels change. In Somali, the terms "Miilaadi" (literally: the Advent, referring to the birth of Christ) and "ka dib Miilaadka" (after the Advent) are also found.