A friend of mine, “Ted,” posted the following as his status on Facebook a few days before the New Year:

TED: 2010 is NOT the start of the a new decade! Decades run 1-10, centuries 1-100, and millenniums 1-1000. Remember there was no year 0000. Happy New Year to all.

We’ll just chalk the semantically confusing “the a new decade” up as a typo. He and I had had a similar conversation before, and I’d pointed out that– heck, here are my comments and his rebuttals:

VDV: 2010 is not the beginning of the second decade of the 21st century, but it is the beginning of a decade–the 2010s. Look at the 1990s–they began on 1/1/90, not 1/1/91. Or do the 90s not count as a decade? If not, what do we call them? Time periods that just happen to share a tens-digit and just happen to last ten years?

TED: Dom, the decade of the 1990’s was 1991-2000. Therefore the 21st century did not start until January 1, 2001.

[NOTE: VDV added the boldface to Ted’s ridiculous statement.]

Here I am trying to explain that there are two different and perfectly valid ways to count decades, centuries, and millennia, and that these two ways are always going to be offset by exactly one year, and then he claims that the 1990s ran from ’91 to 2000. I was having trouble telling whether (A) he really didn’t understand what I meant, or (B) he saw that he was wrong and was trying to save face because he’d been bringing up this issue so much in recent weeks. My smart-aleck response:

VDV: So 1990 wasn’t in the 1990s? Awesome. When I turn 40, I’ll tell everyone I’m still in my thirties.

TED: When you are 40, you are finishing your third decade. Feel better?

VDV: When I am 40, I will be finishing my fourth decade.

Normally, when arguing with someone who is so clearly incorrect, I’ll just give up and let them suffer the consequences of spouting utter nonsense. However, I couldn’t let that happen to this particular friend—I have far too much respect for him. So, in hopes of preventing him from making foolish-sounding statements like “the decade of the 1990’s was 1991-2000,” I sent the following e-mail:

At risk of beating a dead horse…

First, please note that you are correct in suggesting that 1/1/2010 is not the beginning of the second decade of the 21st century, just like 1/1/2000 was not the beginning of the third millennium, or of the 21st century, or of the first decade of the 21st century.

However, 1/1/2010 is the beginning of the decade that we will call the “teens” or the “tens,” just like 1/1/2000 was the beginning of the decade that we call the “zeroes,” or the “aughts,” or the “double-o’s,” or whatever, and was the beginning of the century that we call the “two thousands.”

I tried to point out that 1/1/1990 was the beginning of the decade we call the “nineties,” but it was not the beginning of the final decade of the 20th century. You said, and I quote, “Dom, the decade of the 1990’s was 1991-2000. Therefore the 21st century did not start until January 1, 2001.” Your statement is a non sequitir, because you’ve conflated two things:

1. an ordinal counting of years (the first decade was A.D. 1-10, the first century was A.D. 1-100, the first millennium was A.D. 1-1000), and

2. a nominal sorting of years (the “nineties” were 1990-1999, the “nineteen hundreds” were 1900-1999, etc.).

The ordinal groups of time do not correspond perfectly to the nominal groups. The 21st century (ordinal) is not the same as the 2000s (nominal), but there’s a ninety-nine year overlap. The last decade of the 20th century is not the same as the 1990s, but there’s a nine year overlap.

Therefore:

1/1/2010 is the beginning of a new decade in the sense that we nominally sort decades.

1/1/2011 is the beginning of a new decade in the sense that we ordinally count decades, with 1/1/0001 as the first day of the first ordinal decade, century, and millennium.

If that doesn’t clear it up, I don’t know what will. Happy New Decade anyways!

(I have to thank my friend DFJ3 for offering what I think is a perfectly logical explanation for the difference between the two counting methods.)

Ted’s response:

We’ve had this conversation before.

I tried. Happy New Decade!