It works like this...
End of Year 1 AD - ooh, a year since Jesus was born (hypothetically).
End of Year 2 AD - ooh, 2 years since Jesus was born.
End of Year 9 AD - ooh, 9 years since Jesus was born.
End of Year 10 AD - ooh, 10 years since Jesus was born. End of the the first decade! Yay!
End of Year 2010 AD - end of the 201st decade. Anybody? Anybody?
Now we can quibble about the days that got dropped from the calendar yadda yadda, but from a sheer I-can-count-to-2000 correctness point of view, I'm bound to mark what is (no matter how much less fun it is to see a 0 turning into a 1 than than seeing a 9 into a 0), the passing of another decade.
I'll save the my-how-the-year's-flown-by stuff for another post.