An abbot and mathematician, Little Dennis lived in Rome a generation after Romulus Augustus was ousted as the last emperor. In those days, the eternal city was a shattered husk, its walls breached, its aqueducts mostly cut, its once bustling streets empty and filling in with dust. The only district with a pulse still beating lay across the Tiber, at St Peter's, where Dennis spent his career penning church canons and tinkering with time, a man his contemporaries considered brilliant, but in his letters seems humourless and stodgy.
In 525 Pope John I asked him to calculate dates for upcoming Easters. This was a complicated task, given the formula adopted by the early church, that Easter shall fall on the first Sunday after the first full moon after the Spring Equinox. But Dennis took on the challenge, and dutifully studied the positions of the moon and the sun, and created a chart of upcoming Easters, beginning in 531. Except that the Romans of Dennis's day did not use the year "531". To them this was the year 247, based on a calendar that started with the first year of the reign of Emperor Diocletian, in what we call AD 284.
This is where droll Dennis had his inspiration. Instead of using a timeline that he said glorified a notorious persecutor of Christians, he calculated his dates based on the birth of Christ. Writing to a friend, Dennis says that he "preferred to count and denote the years from the incarnation of our Lord, in order to make the foundation of our hope better known". His preference appeared on his new Easter charts, which began with anno Domini nostri Jesu Christi DXXXI, year of our Lord Jesus Christ 531, or AD 531 for short.
Unfortunately, Dennis made a critical error. He got his dates wrong. For Christ was most likely not born in what Dennis called AD 1. No one knows Christ's true birthday, since those gathered at the manger that day failed to jot down a date. Yet the Gospels provide a few clues, such as Matthew's story of Herod the Great ordering the slaughter of all first- born children under two. Herod died in 4 BC, placing Jesus's birth in at least that year. Other sources offer vague indices that suggest other dates, though most scholars lean toward 4 or 5 BC. If this is true, then the real anno Domini 2000 may have come and gone in 1996 or 1997, and the new millennium is already here.
To add to the confusion, Dennis started his timeline with year one, not zero, for the simple reason that the concept of zero had not yet been introduced in sixth- century Europe. It would not wind its way west from India and the Islamic Near East for several more centuries.
By adding 2000 to the year one, we get 2001, not 2000, as the true start of the millennium. Not that this means anything, since most of earth's 6 billion people plan to make merry on 1 January 2000, a date that is technically incorrect.
Dennis never suspected that he had made the blunder of all time. Neither did anyone else in Europe, where his dating scheme was mostly ignored for centuries. Britain's Venerable Bede was one of the first to use it, in his Ecclesiastical History of the English People, published in 731. Still, the scheme did not become widely used until the eve of the second millennium, in the 10th century.
But let's not berate poor Dennis for making a muddle of the millennium. Nor should anyone cancel their plans to voyage to the Nile, or to make merry at Trafalgar on New Years 2000, or 2001, or both. For, as the old saying goes, all good things will come in time, whenever that may be.
David Ewing Duncan is author of `The Calendar: the 5000-year struggle to align the clock and the heavens, and the missing ten days' (Fourth Estate, pounds 12.99)