I have seen this question asked (or its fraternal twin, “when does the new decade begin”) in a number of places, and my response is this:

That’s a lousy way to phrase the question.

I will remind folks that I work at the Naval Observatory, but this blog in no way represents the official position of USNO. That said, I am one of the people who might be consulted on such a question, if it were difficult to figure out. But it wouldn’t get that far. This is not a “math is hard” subject.

The question is ambiguous, which is why it’s a lousy way to ask it. There is no “the decade” because there are a number of ways to label decades. A decade is a period of 10 years. 2010-2019 is a decade (we would call that the 2010s), as is 2011-2020 (the 202nd decade). The latter is based on numbering the decades, as we tend to do with centuries and millennia, which started with the year 1. The former is a count of an interval, which is a perfectly cromulent way to keep track of things —if we have a meeting on Wednesday and we say “see you in a week” we start counting from that point, not from Sunday (the equivalent of starting at 1). We can use time labels as an interval. We do it all the time. Almost nobody gets confused by this.

Which means the immediate response to the question in the title should be “to which decade are you referring?”  Without that clarification, you get the mess that I’ve been seeing. People assuming one protocol, and, what’s worse, denying that there is another. Instead they blindly point to an authority they claim will back them up, citing the Naval Observatory and the Farmer’s Almanac, except…

The few people who have provided a link sent me to discussions of when the 2nd millennium/20th century ended; some are leftover from these discussions from 20 years ago. Nothing about decades. No discussion of intervals, because when the 2nd millennium/20th century ended has already defined the discussion in a way that “the decade” has not.

The Farmer’s Almanac citation may be from this NPR piece

We ran a story several years ago. In fact, you know, remember the big celebration in 1999. People thought that the new millennial (sic) was going to start the next year. But really, a decade begins actually with the year ending in the numeral one. There was never a year zero. So when we started counting time way back when, it goes one through 10. So a decade is 10 years.

See the problem? It starts with the millennium discussion and applies it. That last sentence — a decade is 10 years — is true and has nothing to do with the bit that preceded it. The fact that there was no year zero has nothing to do with using decade as an interval of time.

NPR does better here

…the definition of a decade is just any 10-year span. Where it begins is fairly arbitrary, in their view.

…

Rick Fienberg of the American Astronomical Society says he doesn’t think his group has adopted an official position on the matter — but he adds, “History is clear: Because there was no Year Zero, the first decade of the common era (CE or AD) was years 1 to 10, the second decade was years 11 to 20, and the next decade will be years 2021 to 2030.”

Part of the issue, he and others say, is our use of “decade” as a frame of cultural reference.

“The reason people get confused is because we tend to think of decades as ‘the 20s’ or ‘the 30s,’ Fienberg says. “It’s true that ‘the 20s’ — that is, the period 2020 to 2029 — is a decade, i.e., 10 years“

Fienberg lays out both cases for you.

The biggest mistake in all of this is people saying that the teens (2010s) don’t end for another year. They have taken both conventions and mashed them together into a horrible, horrible mess. You can talk about the 2010s decade, which ends (as I write this) tonight, and you can talk about the 2nd decade of the third millennium (or any similar numbering convention) which started in 2011 and doesn’t end for another year.

If the US founding fathers had settled on 5-year terms for presidents, it’s quite likely that we in the US would be referring to presidential decades for two-term presidents. Since Washington was sworn in in 1789, we would have these decades for two-termers starting on 9s and 4s (since not all presidents served two terms). And maybe there would be slightly less confusion about such a relatively simple concept.

Pick one convention or the other, as appropriate, and be consistent. Recognize that the other convention exists. And ask better questions.