If you’ve been at all present on the internet and social media in particular during the past few weeks, you’ve probably noticed people arguing about when the new decade actually begins: January 1, 2020 or January 1, 2021.

Culturally, most people agree that decades begin on years ending with zero and end on years ending with nine. The ’70s ran from 1970 to 1979, the ’80s went from 1980 to 1989, and so on. But a swell of people says that way of thinking is incorrect. Because there was no year 0, they argue, it’s more accurate to say that decades begin on years ending with one and end on years ending with zero.

That argument is based on the Gregorian calendar, which is oriented around the birth of Jesus Christ. In the 6th century, an abbot named Dionysius Exiguus attempted to calculate the year Jesus was born. We now know that his calculation was incorrect, but it became the basis for both the Julian and the Gregorian calendars. According to TimeandDate.com, Dionysius labeled the year of Jesus’s birth with the Roman numeral for one (I). About 200 years later, a monk known as St. Bede the Venerable completed five books of history that calculated the years before Christ’s birth. But Bede recorded the latest year before Jesus’s birth as 1 B.C., leaving no space for a year 0. In fact, the concept of zero didn’t even reach Europe until at least the 12th century.

So technically, it’s only been 2019 years since the year Dionysius determined was 1 A.D.—not 2020. That’s led some organizations, like the Farmers' Almanac and the United States Naval Observatory, to favor a decade that begins in 2021 and ends in 2030.

That’s not the whole story, though. Andrew Novick, an electrical engineer who works for the National Institute of Standards and Technology, told NPR that it’s mainly a semantic issue.

“The definition of a decade really is just 10 years,” he said. So 2020 to 2029 works just as well as 2021 to 2030. But because many people tend to discuss decades in terms of culture, it gets confusing when you decide that the year 1990 was technically in the same decade as the ’80s, Novick said. “I would say however somebody wants to define something in language is up to them,” he told NPR. “But they might have to clarify how they’re defining it so that people know what they’re saying.”

TimeandDate.com points out that this specific issue only exists with the widely used Gregorian calendar, and that other calendars—like the Jewish calendar and the Islamic calendar—use completely different numbers for years.

“It’s a man-made system,” Konstantin Bikos, the lead editor of TimeandDate.com, told CNN. “It matters in terms of categorizing time spans and talking about time spans. But the 203rd decade is, in astronomical terms, no different than the decade before or after.”

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