other · 525 · Rome
The Alexandrian Easter tables in use at Rome numbered their years from the accession of Diocletian, the emperor of the last and worst persecution — the Era of the Martyrs. When Dionysius, a Scythian monk living in Rome and one of the few men of his day fluent in both Greek and Latin, was asked to continue the tables past 531, he said he was unwilling to perpetuate the memory of a persecutor and would count instead from the incarnation of Jesus Christ. He put his own first year at 525 and worked backwards. That is the whole origin of the era: a preface to a table of Easter dates, with a moral reason given for it. He was wrong by some years — Herod died in what this reckoning calls 4 BC, so the birth it is counted from falls before its own year one, which is why the dates on the earliest pages of this dataset look the way they do. It spread slowly, through Bede above all, and took until roughly the eleventh century to become normal in the west. His other work was the first great Latin collection of canon law and papal decretals, the Collectio Dionysiana, which is why the councils this dataset records reached the medieval west in the shape they did.
Why is this year numbered from where it is numbered?
Because one monk would not go on counting from a persecutor.