Dionysius Exiguus, d. c.545, Roman monk, chronologist, and scholar, a transmitter of Greek thought to the Middle Ages. He made collections of 5th-century papal decretals and the canons of the early church councils. Dionysius, in an attempt to improve the reckoning of the date of Easter, was the first (525) to use our present system of reckoning a date from the time of the birth of Jesus (see era).