Quite by accident, just the other day I found myself embroiled in a controversy on Twitter of my own making. I made an offhand mention that the decade would be ending in a few short days. That seemed obvious enough to me, but apparently not so to many others. What ensued was something of a firestorm of people, many of whom disagreed with me. However, I maintain that I was right all along. Here’s the scoop.

My claim is that December 31, 2009 — today, as this is posted — is not just the last day of the year, but the last day of a decade. Now, I don’t mean that in the trivial sense that any moment is the last moment of the past ten year period — you can always talk about the last ten years that end at any time.

I meant, and still mean, specifically the first decade of the 2000s. That does in deed and in fact end today.

What people were arguing over were things like centuries and millennia, and how there was no year 0, and therefore the last day of the decade is actually December 31, 2010. But that’s not relevant because we don’t measure decades the same way we do centuries.

Certainly, the last day of the 20th century was December 31, 2000. In that case, there was no year 0, so the first year of the 1st century ended on December 31, 1 A.D. Doing the math, it’s easy to see that 1999 more years needed to elapse to end the 20th century, and so its demise was on that last calendar day of 2000. January 1, 2001 marked the first day of the 21st century.

But we don’t reckon decades like that. We refer to them by the tens place in the year’s numerals: the 70s, the 80s, the 90s. And since we do, clearly, today is the last day of the decade we will call the aughts or zeroes or whatever.

Actually, looking at this now, it seems to me that centuries are more formal, with an actual method of naming them, whereas decades are more of a nickname, a handy handle to use when referring to a time period.

Also, you wouldn’t say that 1990 was part of the 80s, would you? I think it’s clear that December 31, 1989 was the last day of the 80s, just as December 31, 2009 is the last day of whatever term we’ll wind up using to refer to the first 10 years of the 2000s.

Confusing this a bit is that we might refer to something happening in the 1900s versus saying it happened in the 20th century. Those terms are synonymous, barring the year of 1900, which was in the 19th century, and 2000, which was in the 20th century but not in the 1900s.

If we did reckon decades the same way as centuries then a point would be made that the decade ends in 2010. But we don’t, and in this case there truly is a year 0: the year 2000. So once again, the first decade of the 2000s ends today.

A couple of people pointed out that this means the first decade in our calendar only had 9 years: AD 1 – 9. I suppose that’s true, and so it’s not really a decade then in the strict definition of the word. But since we’re not using a rigorous naming convention, and references to decades are more like nicknames. Plus, who talks about the first ten years of our calendar that way anyway?

Confusing this even more was the case someone made that when you are 30, you no longer say you are in your 20s (unless you’re lying). But all during that last year, when you say you are 29, you are actually living your 30th year on Earth. After all, when we say a baby is 1, really they have already been around 12 months. We change the number after the fact, so when you turn 30 you’ve already lived out your 30th year. The whole time you are 29, you’re plowing through your 30th year.

Perhaps it would lessen the issue if, when asked how old you are on your birthday, instead of saying "I am 30," you say "I have just completed my 30th year." I suspect that won’t catch on, however.

Still, be all that as it may, when you are 29 you are still in your 20s, and when you turn 30 you ain’t.

The lessons here are many fold. One is that, and pardon my repetition, the first decade of the 2000s ends today. A second is that people are still terribly confused about how to delineate centuries. A third is that this can be generalized to people being confused on how we delineate time.

Fourth is that this is all arbitrary and a bit silly. But we do make rules, and sometimes those rules have to make sense, and sometimes it’s fun to talk about them even when it means some people disagree.

And fifth? People shouldn’t argue with me on Twitter. At least not until the next decade starts.

And if I may indulge myself, one final thing:

Happy new year!

And happy new decade. May the 10s and teens treat us all better — and may we make them better — than the aughts.