Dionysius Exiguus was a 6th-century Eastern Roman monk born in Scythia Minor and active in Rome. He was a member of a community of Scythian monks concentrated in Tomis, the major city of Scythia Minor. Dionysius is best known as the inventor of Anno Domini (AD) dating, which is used to number the years of both the Gregorian calendar and the (Christianised) Julian calendar. Almost all churches adopted his computus for the dates of Easter. 475 – 550; died at about 75. Alive at the same time as Justinian I, Aryabhata, Boethius, Gregory I, and Benedict of Nursia. Wiki link