I was curious why you were asking so I thought I’d quickly find an answer with a definitive source but this ended up being much more controversial than I though

I was curious why you were asking so I thought I’d quickly find an answer with a definitive source but this ended up being much more controversial than I thought. The popular answer seems to be a decade starts and ends with the decade handle. 200X, 201X, 202X, 203X, 204X. The current decade began when it was 2020 and will end when it’s no longer 202X. https://www.npr.org/2019/12/27/791546842/people-cant-even-agree-on-when-the-decade-ends https://www.npr.org/2019/12/27/791546842/people-cant-even-ag... > As Jan. 1, 2020, approaches, it turns out there is a Team Zero and a Team 1 – those who believe the new decade will begin after midnight on the upcoming New Year's Eve and those who believe the burgeoning celebrations of a new decade (and all the "last decade" retrospectives) are in fact a year early. > In a recent YouGov survey, 64% of Americans said the next decade will begin on Jan. 1, 2020, and end on Dec. 31, 2029. But nearly 20% said they weren't sure – and slightly fewer people said the next decade won't start until Jan. 1, 2021. > On this question, many voices of authority are with the minority. They say that because there was no Year Zero when the current era began more than 2,000 years ago, all decades, centuries and millenia begin with Year 1.