There has been much debate about when the old decade ends and the new one begins. Some say the old decade ended on December 31, 2019, and the start of the new one began January 1, 2020. For others, the new decade doesn’t start until January 1, 2021; the old one concluding on December 31, 2020.
But which is correct?
As you think about New Year’s resolutions, here’s one we should all make together: resolve to insist that decades begin with the year ending in the numeral 1 and finish with a 0. For a decade to begin, we must start with the year ending with 1 (2021) and finish with 10, or so far as chronology is concerned, a year ending in 0 (2030).
For example, January 1, 2001, opened the 21st century and the start of the new millennium, just as the year 1 A.D. marked the beginning of the Christian era. Of course, many of us will remember the wild celebrations that were touched off at midnight on December 31, 1999. But was that a year too soon? Yes!
That fact was known even to comedian Jerry Seinfeld. And if at the stroke of midnight on December 31, 2019 you thought you were celebrating the start of a new decade, guess again. As was the case 20 years ago, you’ll be one year early, for the new decade will actually start in the year 2021.
If you want to criticize anybody for this confusion, you can point the finger of blame at two men: Dionysius Exiguus, also known in some reference works as “Dennis the Short,” and the Northumbrian monk Bede, also known as the “Venerable Bede.”
Dionysius was born in what we now call Romania around the year 470 and was the first to suggest counting the passage of the years from the date of the birth of Jesus Christ; the beginning of the anno Domini (which means “Year of Our Lord” in Latin) era, or A.D.
According to the contemporary historians of the time, Jesus was born during the 28th year of the reign of the Roman Emperor, Caesar Augustus. There is, however, considerable confusion about exactly when Augustus’ reign began, so the year Dionysius called 1 A.D. was not accurately placed in history; in fact, most religious scholars now think that Jesus might actually have been born several years earlier.
When Dionysius finished his computations, he figured that the year Christ was living in was 525 A.D. But he never bothered to number the years prior to Christ’s birth.
We would have to wait until 731 A.D. when the Venerable Bede popularized the anno Domini era in Anglo-Saxon England and extended the counting of years before the birth of Christ – the “B.C. era.”
Most unfortunately, however, Bede did not account for the year zero in his calculations. So 1 A.D. was immediately preceded not by a 0, but by 1 B.C.
The Elevator Analogy
As an analogy, think of going into a building in which the ground floor is listed not as the first floor, but as the lobby. So the first floor is actually one flight above you.
So if you were to go into an elevator located in the lobby and wanted to go 10-flights up, you would actually end up on the ninth floor (if you were to assume that the lobby as the “zero” floor).
But if you assume the lobby as the “first” floor and went 10-flights up, you would end up on the tenth floor.
In essence, on our calendars, 2021 is the equivalent of a “first-floor lobby,” and after going up ten flights (or years), we’ll arrive at the tenth floor. Or in this case, the year 2030—when that decade ends.
Fun Fact: The origin of the word decade goes back to the Greek word Deka meaning ten and dates from the early 17th century.
Joe Rao is an esteemed astronomer who writes for Space.com, Sky & Telescope, and Natural History Magazine. Mr. Rao is a regular contributor to the Farmers' Almanac and serves as an associate lecturer for the Hayden Planetarium in New York City.