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?

Let’s dive in, shall we?

It depends on who you ask

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.

In 525, a monk known as Dionysius Exiguus set out to determine the date of Easter and created a system of labeling years based on the date he thought Jesus Christ was born (a date that’s considered historically inaccurate today). But he didn’t account for the years before the birth of Jesus.

That was done in 731 by a monk known as the Venerable Bede. Bede counted the years before Christ and established the BC era, but he didn’t include a year zero in his calculations. Which means that the year before 1 AD was 1 BC.

Because there’s no year 0 in the calendar, the first year was complete at the end of year 1, not at its beginning.

By that same logic, the first decade in the calendar was complete at the end of 10 CE, or 10 AD.

Which means, according to the Farmers’ Almanac, that the end of this decade is December 31, 2020, not December 31, 2019.

We talk about decades differently

Those technicalities, however, don’t change the fact that as a society, we seem to have collectively determined that decades begin in years ending in zero and end on years ending in nine.

After all, it makes sense.

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.

So when it comes to decades, Time and Date is flexible.

This definition of a decade is so ingrained in popular culture that the people at Encyclopaedia Britannica never really considered the possibility that a decade could begin in any other year besides one ending in zero.

“I love the question because it has never really occurred to me or any of us at Britannica to be concerned about it,” J.E. Luebering, executive editorial director at Encylopaedia Britannica, told CNN. “That may underscore that we’re humanities-leaning perhaps than science-leaning. But the consensus here is that we’re a bit mystified by the debate.”

Does any of this matter?

Astronomically, not really.

The only thing that year numbers really signify is the number of times that the Earth has completed a full revolution around the sun.

“In terms of counting millennia or centuries or decades, it doesn’t really matter,” Bikos told CNN. “It’s a man-made system. 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.”

Others point out that the system on which our calendar is based is riddled with inaccuracies and inconsistencies, making the whole discussion arbitrary.

“In the grand scheme of things, it seems like a lot of technicalities on something that hasn’t been very consistent,” Luebering said.

Time and Date adds that the debate about the new decade is only relevant right now in the Gregorian calendar. Though that’s the calendar that most Western countries abide by, there are a bunch of other calendars that use entirely different year numbers, like the Jewish calendar, the Islamic calendar, and the Hindu calendar.

“So, while the Gregorian calendar is the system officially used around the world, this goes to show that our year count is nothing more than a random fabrication, which is ultimately based on the ideas and religious fervor of a 6th-century monk,” Bikos wrote in an article about the start of the 21st century.

And that year count is based on a date that scholars now believe is wrong.

“What’s more, Dionysius Exiguus based the beginning of year BCE 1 in the Julian calendar, the predecessor of today’s calendar system, on a religious event—the birth of Jesus—which not only lacks astronomical relevance but is also based on religious lore and, as such, a rough estimation at best.”

So there you have it, folks. Time is an illusion.