BC and AD

The year numbering system used with Common Era notation was devised by the monk Dionysius Exiguus in the year 525 to replace the Diocletian years, because he did not wish to continue the memory of a tyrant who persecuted Christians. He attempted to number years from an event he referred to as the Incarnation of Jesus, although scholars today generally agree that he miscalculated by a small number of years. . . . Numbering years in this manner became more widespread with its usage by Bede in England in 731. Bede also introduced the practice of dating years before the supposed year of birth of Jesus, and the practice of not using a year zero. In 1422, Portugal became the last Western European country to switch to the system begun by Dionysius.

http://en.wikipedia.org/wiki/Common_Era

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