Dionysius Exiguus

Dionysius Exiguus (Dennis the Small, Dennis the Little or Dennis the Short, meaning humble) (c. 470 – c. 544) was a 6th-century monk born in Scythia Minor, modern Dobruja shared by Romania and Bulgaria. He was a member of the Scythian monks community concentrated in Tomis, the major city of Scythia Minor. Dionysius is best known as the inventor of the Anno Domini (AD) era, which is used to number the years of both the Gregorian calendar and the (Christianized) Julian calendar.

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