Marc Theisson posts:

2010 is the last year of the decade. The new decade begins in 2011!

Some dude named Phil Plait weighs in:

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.

And then this guy thinks decades end at the end of xx10 because when you count, you count from 1 to 10:

Quick. Count to 10.

OK, now think back. When you were counting, what was the first number you used? What was the last?

That’s right. You began with “1” and you ended with “10.” That is something that we know before we begin first grade.

As a math-y, computer science-y kind of guy, I have to point out: accurate counting begins with zero.  By that light, the fact that there was no year “0″ is an error in our calendar system, not a feature.  Why then would we want to perpetuate the mistake throughout all time instead of just calling it a 9-year decade, and a 99-year century and be done with it?  The people who lived in those times are long dead and don’t care anyway.

Update: Fred Schwarz chips in -

…there’s no “technically” or “officially” about it. A decade begins or ends whenever you want it to, and one system is as right or wrong as another…The lengths of a day and a year might seem to be unalterably fixed by nature, but in fact they are not…So if the length of a day, a month, and a year can vary, why not the length of a decade? Why not define the first decade A.D. to be nine years long, ending in the year 9, and follow the natural 0-to-9 pattern from then on?

Exactly right, but you heard it here first.