Oh, to revel in New Year’s Eve. Cheer with good friends! A glass of Champagne or two. And, this year, ringing in a new decade.

Oh, wait. Are we?

In recent years there has been debate about when a decade begins and ends. For some people, the next decade will begin on Jan. 1, 2020, and end on Dec. 31, 2029. For others, it won’t start until Jan. 1, 2021, concluding on Dec. 31, 2030. To the average person — well, many of us — it can be very confusing. But what is correct? Should we celebrate the beginning of a new decade now or wait until next year, when it seems a little passé?

We decided to break it down for readers. We talked to an expert at the United States Naval Observatory, which runs the nation’s master clock. We interviewed an editor at the Farmers’ Almanac. We even talked to a curator of astrophysics.

There is one thing almost everyone agreed on. “Talking about it is a great way to get into a bar fight,” said Geoff Chester, an astronomer and a public affairs officer at the observatory.