celebrated 6th-century canonist who is considered the inventor of the Christian calendar, the use of which spread through the employment of his new Easter tables.

The 6th-century historian Cassiodorus calls him a monk, but tradition refers to him as an abbot. He arrived in Rome about the time of the death (496) of Pope St. Gelasius I (Gelasius I, Saint), who had summoned him to organize the pontifical archives. Thereafter, Dionysius flourished as a scholar at Rome. In 525, at the request of Pope St. John I (John I, Saint), he prepared the chronology still current; it was a modified Alexandrian computation (95-year tables evolved by the patriarch Theophilus of Alexandria) based on Victorius of Aquitaine's 532-year cycle. He wrongly dated the birth of Christ according to the Roman system (i.e., 754 years after the founding of Rome) as Dec. 25, 753.

Highly reputed as a theologian and as an accomplished mathematician and astronomer, Dionysius was well versed in the Holy Scriptures and in canon law. Credited to him are a collection of 401 ecclesiastical canons—including the apostolic canons and the decrees of the councils of Nicaea, Constantinople, Chalcedon, and Sardis—and a collection of the decretals of the popes from St. Siricius (384–399) to Anastasius II (496–498). Dionysius also translated many Greek works now lost, including a life of St. Pachomius and an instruction of St. Proclus of Constantinople.