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When does the decade begin and end anyway?

Every four years in the Gregorian calendar, an extra day is added to the calendar in order to synchronize it with the solar year.
Every four years in the Gregorian calendar, an extra day is added to the calendar in order to synchronize it with the solar year. (iStock/Picsfive)

CNN – 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?

When exactly the decade begins and ends all depends on who you ask.

The US Naval Observatory, the agency that maintains the country’s master clock, tackled this question in 1999 as people debated when the new millennium would begin. According to the astronomical dating system through which it measures time, the observatory stated that the new millennium would begin on January 1, 2001.

The Farmers’ Almanac, America’s centuries-old go-to for weather predictions, astronomical data and more, takes a similar position.

"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," an article on its website reads.

The Farmers’ Almanac insists on this because of how years are numbered in the Gregorian calendar, the system in official use throughout most of the world. The anno domini era, or the common era, begins with year 1 on the Gregorian calendar.