At the end of 1999, some people declared that we were about to embark on the 21st century. Those people were mistaken. The 21st century did not begin until 2001. There was no year zero A.D. The first year was 1 A.D.; thus, the first century was 1-100; the second century was 101-200; and the 20th century was 1901-2000. The 21st century didn’t start until 2001, and will last through 2100.
Everyone knows this now, and the people who thought the 21st century would begin in 2000 have accepted their error. You live and you learn. Unfortunately, some people have taken this idea beyond its bounds and declared that the current decade, the 2000s, is NOT about to end. They say it won’t end until 2010.
Here is the problem. We are talking about two different things. The logic used with centuries doesn’t apply to decades because we don’t number our decades the way we number our centuries. Technically, this is the 201st decade A.D. But we don’t call it that. We call it the ’00s. The ’00s are, by definition, the years that have “0” as their next-to-last digit: 2000-2009. Likewise, the ’80s were, by definition, the years that had “8” as their next-to-last digit: 1980-1989. The year 1990 was obviously not part of “the 1980s.” Try saying “1990” out loud. Did you say “eighty” at any point? No? Then it’s not part of “the eighties,” is it?
Now, if we did refer to this as the 201st decade, you would be absolutely correct that it won’t end until the end of 2010. The years 1-10 were the first decade; 11-20 were the second decade; 1991-2000 were the 200th decade; and 2001-2010 are the 201st decade.
But we don’t refer to our decades that way. A “decade” is ANY sequence of ten years. 1984-1993 was a “decade.” The people who said “the century” was ending in 1999 were right — the “century” of 1900-1999 was indeed ending. And in 1985, the “century” of 1886-1985 was ending. They were only mistaken when they said “the 20th century” was ending. “The 20th century” is specific and defined, and can only mean the years 1901-2000.
In common parlance, when we talk about what “decade” it is, we’re talking about which digit is next-to-last in the year number. If that digit is a 9, that year was part of the ’90s. If that digit is a 0, it’s part of the ’00s. That’s how we do it in our culture. Most of the time, when we talk about the “decade,” that’s the criteria we’re using. The whole “there was no year zero” thing has nothing to do with it. That would only matter if we were counting the number of decades, which we don’t do. That’s why you’re seeing “best of the decade” lists now, at the tail end of 2009: because 2010 will be part of a new “decade,” the ’10s. Get it?
P.S. I thought of another example that may help clarify this important subject in the mind of the reader. When you talk about “the 1900s,” you mean the years 1900-1999 — the ones that have “nineteen” in them. At the end of 1999, “the 1900s” were indeed ending; the 20th century, however, was not. It is the same principle with the decades, the only difference being that we do not commonly number our decades like we do our centuries.