When does the new decade begin?

There has been some discussion among some people around my office as to when the new decade begins: January 1, 2010, or January 1, 2011. Someone finally asked me… they probably gave up on getting a simple, short answer that would persuade them, so they went to mister have-you-got-five-minutes. Here’s my disquisition:

Q. What was the first year AD? Zero or one?

A. AD 1. Before that was 1 BC.

Q. Why does that matter?

A. Well, so what was the first decade AD? One, two, three, four… A decade is ten years. The first ten years were completed at the end of the year AD 10. The first century was completed at the end of the year AD 100. The second millennium – the end of two thousand years since the point at which we started counting forward – was the end of AD 2000.

Of course, a lot of people thought that the turning of the millennium was when it turned from 1999 to 2000. And no surprise that they did: aside from the obvious change-over of the numbers, the way we count years of age is just the opposite. When’s your first year of life? Why, the year before you turn 1. Once you turn 1 you’ve finished your first year. Once you turn 2 you’ve finished your second year. When you’re 39 you’re in your 40th year.

So here’s this great irony: people keep saying someone who’s 40 is in their 40th year, which is wrong, but they also keep thinking that, for instance, the second millennium AD ends at the beginning of AD 2000, not the end, which is also wrong for the exactly converse reason. So we number years the way we count objects: the first one is 1, from beginning to end; the second, starting right after that, is 2; and so on. But we measure age the way we measure other things: you don’t get to 1 until you actually have a whole one.

Q. OK, fine, but just say it: when is the start of a new decade?

A. If you’re counting decades since the beginning of the counting of years, each decade ends at the end of the -0 year and begins at the beginning of the -1 year. So the current decade, if you’re making stacks of 10 like poker chips starting with AD 1, ends at the end of 2010.

But! The way people think about and refer to decades in our culture is different! A decade is just any set of ten years, after all. I can refer to the decade 1984–1993 if I want (not 1984–1994 unless I exclude either 1984 or 1994 from the counting). And when people refer to the seventies, they mean 1970 through 1979. That just happens to be the way we think about them. And since we really didn’t start thinking about them that way two thousand years ago, it doesn’t actually matter that by this way of thinking the first decade AD only had 9 years. (Jesus wasn’t born in AD 1 or 1 BC anyway. It’s now thought he was born 4 or 6 years earlier.) So the decade that people are trying to decide whether to call the zeros or the oh-ohs or the naughties (naught=0) or whatever is the decade including all years 200x. That does include the last year of the second millennium, technically, but you know what? It can. It’s still a decade. And the decade I feel like calling the onesies starts January 1, 2010.

So in the end, it really depends on what your purpose is. The real truth, as I said, is that a new decade starts whenever you want it to. The time starting exactly now and ending in exactly 10 years is a decade. So pick your frame of reference. If you want to refer to a decade such as we usually think of them in our culture, the ’20s, ’40s, ’70s, etc., then 2010 starts a new decade. If you want to play little gotcha-games, you can say, “Oh, no, the first decade of the third millennium isn’t over until the end of 2010!” Both are true.