From 500 he lived as monk and friend of Cassiodorus (who wrote about him in Institutiones) in Rome where, as an abbot and learned member of the Vatican's Curia, he translated from Greek into Latin 401 ecclesiastical canons, including the apostolical canons and the decrees of the councils of Nicaea, Constantinople, Chalcedon and Sardis, and also a collection of the decretals of the popes from Siricius to Anastasius II. These collections had great authority in the West and still guide church administrations. He was known for his mathematical skill and for being versed in astronomy, as well.
Pope John I requested that Dionysius compute a table for future dates of Easter. In 525, Dionysius produced his Liber de Paschate. It starts with a letter to a bishop Petronius introducing the work. Then follow the tables and an explanation on how to use and compute them. Letters from the Alexandrian bishops to Pope Leo, and by Dionysius form appendices to this work.
The previous tables had been based on a method by the Alexandrian bishops Theophilus and St. Cyrillus, which covered a period of 95 years. The first part of Dionysius tables covered the last 19 years of the running 95-year period, and had the years labeled according to the era of the accession of Roman Emperor Diocletian (August 28, September 29, or November 17, 284 - sources disagree on the exact date), as had been the custom since the First Council of Nicaea (325); this table continued up to the year 247. As Dionysius explained to Petronius, he did not wish to continue the memory of a tyrant who persecuted Christians, and instead he proposed to number years from the incarnation of Jesus Christ, starting his new 95-year table with the year 532 .
Since the 2nd century some bishoprics in the East of the Roman Empire counted years from the birth of Christ, but there was no agreement on the correct epoch; Clement of Alexandria (ca. 190 AD) and Eusebius of Caesarea (ca. 320 AD) wrote about these attempts. It is not clear how Dionysius fixed the beginning of the Christian era. Before him, Victorius of Aquitania devised a method to compute Easter which had a cycle of 532 years. However, Dionysius used a modified version of the Alexandrian method, and does not mention Victorius or the 532-year cycle. In any case, the previous 532-year period would have started in 1 BC (Dionysius count). The text as it has been handed down to us mentions the date for Christ's conception as Sunday March 25, and his birth as Tuesday December 25, presumably in the year preceding the beginning of Dionysius' era. He equates these and other events with the dates of equinoxes and solstices in Jesus' time. However it is not clear where Dionysius puts the beginning of the year: besides January 1, also March 1, Christmas day (December 25), Easter day, and the date of the vernal equinox (March 21 or even March 25) are "styles" that have been used at one time. From what we know of the historical context, Jesus is believed to have been born some years before the date that Dionysius gives.
Promoted by the Venerable Bede, the year numbering system of Dionysius spread during the Middle Ages. Bede himself seems to have instituted the "BC" and "AD" year naming convention.
Dionysius's definition of Easter date (per the First Council of Nicaea): Easter is the Sunday following the first Luna XIV (the 14th day of the moon) that occurs on or after XII Kalendas Aprilis (21 March) (kalendas means the first day of the month, and the date given counts days backward starting with 1 on the first day of the given month, which is according to the Roman custom). The change from 15 Nisan of the Jewish Pesach to Luna 14 probably has to do with the fact that on the Hebrew calendar days start at sunset, while in the Christian A.D. calendar (which was also introduced by Dionysius) days start at midnight. Dionysius's method of computing Easter is called the Julian method.