Is 2020 the start of a new decade?

By Joel Shannon

The upcoming New Year has been widely heralded as the start of a new decade — the 2020s. But an old timekeeping dilemma has led some to contend we’re celebrating too early and should actually wait until January 1, 2021.

A decade is a period of 10 years — that isn’t disputed. But a glitch in our calendar system creates a problem if you think decades should begin in years ending with a zero. If you continue the pattern back about 2020 years, you run into a major issue.

That’s because there is no “year zero” in our calendar system, explained Craig Callender, a professor of philosophy at UC San Diego who has studied the physics and experience of time.

The lack of a year zero means the only consistent way to measure decades — or centuries or millennia — is to start them in years ending in one: That’s the theory presented in explanations of the debate published by the Farmer’s Almanac, timeanddate.com, the New York Times and others.

But because a decade is simply a unit of 10 years, there is no right answer, Callender told USA Today.

Here’s the argument explained:

The case for beginning the decade in 2021

The basis of this argument is a single calendar year that happened more than two millennia ago.

At that point in time, our current calendar system pivots from years labelled BC — “Before Christ” — to years labelled AD — “anno Domini” or “in the year of the Lord”. It’s an attempt to centre the calendar around the birth of Jesus, although the historical accuracy of that date has been called into question.

There is no year zero in the switch, Callender said. The BC years count down to 1 BC, which is immediately followed by AD 1.

The lack of a year zero means the first AD decade started at year 1 and ran until the end of year 10; the second decade started on year 11.

Follow the pattern for 200 more decades, and here we are — still one year away from the start of the next decade, which will begin on January 1, 2021.

The case for beginning the decade in 2020

People who say we should wait a year to celebrate the new decade have a point, but it’s not a very good one, according to Callender.

Marking the beginning of a decade in a year ending in one is out of step with how we think and talk about time, he said.

When we count our children’s ages, we start at zero — not one. When we talk about decades such as the 1960s, we start at years ending with zero — not ending in one.

Celebrating the beginning the decade on Jan. 1, 2020, simply makes things easier in the present day, Callender said. It’s a convention that is “pretty good for everything really, unless you run it all the way back”.

The widely-documented argument between timekeeping purists and the average person is “pretty silly”, he commented. The reality: There’s no perfect solution that will please everyone.