When Does the New Decade Start?

Jeff Jacoby (Boston Globe) writes:

The first decade of the 1st century ran from Year 1 through Year 10. The first decade of the 21st century, therefore, consists of the years 2001 through 2010, no matter how many “Decade in Review” essays, roundups, recaps, and slideshows you’re being bombarded with as 2009 comes to an end.

All this premature enumeration reminds me of a lapel button the late David Brudnoy took to wearing in the last weeks of 1999, amid the frenzied countdown to Y2K and the “end” of the 20th century. “The century will end on December 31, 2000,” it read. “Please be patient.”

Does the new decade really start only in 2011, rather than in 2010?

Well, it all depends on which decade you’re speaking of. Let’s begin with centuries. We’ve set up our BC and AD dating system so there’s no year zero, presumably because back when they were set up, the concept of “zero” as a number was not well-established in Europe. Therefore, the first hundred years of AD time were 1 to 100, the next 101 to 200, and so on. That’s why the 20th century is conventionally understood as going from 1901 to 2000, though I’m sure the term hasn’t always been used entirely consistently.

But of course any consecutive sequence of 100 years can constitute a century. By convention we don’t label 1937 to 2036, for instance, a century. But we do conventionally use another sort of century — centuries labeled things like the 1800s or the 1900s. As I have seen the term used, the 1900s are a century that goes from 1900 to 1999, probably because it would seem quite odd to treat 2000 as part of the 1900s but to exclude 1900 itself from the 1900s. So the “turn of the century” was 2000 if you’re talking about the turn from the 1900s to the 2000s, and 2001 if you’re talking about the turn from the 20th century to the 21st century.

Now as to decades. Indeed, the first decade of the 21st century would logically be 2001 to 2010, and it may be that this term is indeed conventionally used to describe those ten years, though I’m not at all positive that it is. But in any event the usage gets rare past the first few decades of a century. The more common usage, especially past the first few decades, is something like “the 1980s” or “the ’60s.” (As to the first decade, “the first decade” seems to be more common, since “the 1900s” could be understood as either the decade or the century.) And both by logic and by convention (as I understand the convention), “the 1980s” means 1980-89, even if “the ninth decade of the twentieth century” means 1981-1990.

So the second decade of the twenty-first century, if you want to use the term “second decade,” might begin either in the 2010 or 2011; my sense is that actual usage is mixed on this. But the decade that we will call the 2010s (or “the tens” or “the teens,” whichever the preferred term ends up being) will pretty certainly begin a few days from now, on Jan. 1, 2010, and will run until the end of 2019.