Why does our calendar count the years from a monk's guess?

Our years are numbered from a date a monk set in 525 without ever explaining how he got it. Dionysius Exiguus, working in Rome, was drawing up a table of Easter dates. Older tables counted years from the reign of the emperor Diocletian, and Dionysius did not wish to continue the memory of a tyrant who persecuted Christians. So he counted instead from the incarnation of Jesus, stating that the 'present year' was 525 years after it, and he never related that date to any other dating system or justified it.

Historians can only guess at his method. One theory is that he started from the Gospel of Luke, which says Jesus was 'about thirty years old' shortly after the fifteenth year of the emperor Tiberius, and subtracted thirty years. Another is that he counted back 532 years, the length of the cycle after which Easter dates repeat. The historian Georges Declercq has speculated about a motive too: some people believed the world would end 500 years after Jesus's birth, by an older calendar counting from Creation, and a new era might have helped stop people expecting the end.

The system spread slowly. The English historian Bede used it in his Ecclesiastical History of the English People, finished in 731, and the scholar Alcuin brought it to the court culture of Charlemagne's empire; it was in wide use by the 9th century. And the guess was probably off: modern scholars believe Jesus was actually born around 5 BC.