Dionysius Exiguus

Dionysius was born around 470 in Scythia Minor, part of a community of Eastern Roman monks centered in Tomis on the Black Sea. By 500 he'd moved to Rome, where he spent the rest of his life as a monk - not an abbot, despite later honorifics, but a senior and respected member of his community. He translated 401 Church canons from Greek into Latin, codifying the decrees of major councils from Nicaea to Chalcedon; these collections carried such weight they still guide church administration today. He also wrote on elementary mathematics and devised the computus that nearly every church adopted for calculating Easter. But the work that outlasted everything: his Anno Domini dating system, numbering years from the birth of Christ, which became the backbone of both the Julian and Gregorian calendars and the way billions of people, religious or not, mark time.