When Does the New Decade Begin: In a Month, or a Year From Now?

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 Jan. 1, 2020, and end Dec. 31, 2029. For others, it won’t start until Jan. 1, 2021, concluding 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...