Defining a decade

Today is the last day of 2019, meaning that the argument we have every ten years is back again:

As January 1, 2020, approaches, everyone is reflecting about the past decade and the new one that awaits. "Best of the decade" lists are everywhere. #10YearChallenges are all over social media. And people are eagerly gearing up to celebrate the end of the 2010s. 

But there's a slight problem. 

We might be celebrating a year too early, at least according to some people. 

The question of when exactly the current decade ends and the new one begins seems to come up every time the year on the calendar moves from ending in 9 to ending in 0. It came up in 1989. And in 1999. Then again in 2009. And now. 

So is January 1, 2020, really the beginning of the decade? Or does it, in fact, begin a year later, on January 1, 2021?

From a mathematical standpoint, it is correct that the next decade does not begin until 2021. The Gregorian calendar does not have a "Year Zero." The Common Era (Anno Domini) begins with Year 1. Therefore, the first decade of the calendar runs from 1 CE through 10 CE, and all subsequent ten-year spans start with 1 and end with 0, as well.

That being said, from a cultural standpoint, we prefer to group decades by the tens digit, so that they start with 0 and end with 9. It's just easier to categorize years in this manner.

When we think of the 90s, we think of the period from 1990-1999. It just doesn't make sense that the year 1990 would be considered part of the 80s.

Plus, it's more satisfying to celebrate big occasions like the start of a new decade in an even-numbered year, a phenomenon that psychologists call "round number bias." Waiting until 2021 to celebrate the new decade would feel anticlimactic.

That's why Konstantin Bikos, lead editor of TimeandDate.com, says that both definitions of when the new decade begins are correct. No need to cancel your end-of-the-decade party. 

"There's two different ways of categorizing 10 years," he told CNN. "It could be from the year ending in 0 to the year ending in 9, or the year ending in 1 to the year ending in 0."

It comes down to how we talk about time spans. 

Bikos agrees that centuries and millennia always start with years ending in 1. Those time spans are typically referred to as numbered entities counted up from the year 1 AD, as in the "21st century" or the "third millennium." 

Decades are categorized by year numbers. Even though the 2020s will be the 203rd decade, no one ever calls it that. It's just called the 2020s, or the 20s.

Truth be told, this is all very arbitrary. A "decade" could be any ten consecutive orbits of the earth around the sun. 1995 through 2004 is a “decade." Our culture has simply selected years ending 0-9 to be grouped as a decade because it's easier to remember and talk about them that way.

So you will be mathematically correct if you wanted to celebrate the beginning of the 203rd decade, CE, on January 1, 2021. But the "twenties," as they will be known in popular culture, begin at midnight tonight.

Vox reviews the decade just past - what it calls the "tumultuous 2010s" - here.