[aruanan]

But you can have year 2000.

That's because it's the 2000th year. There was, however, no 0th year. You can have a 10th inch on a ruler but the first inch is not the 0th inch.

[rickmichaels]

Any ten year period is a decade. Jan. 1, 2000 to Dec. 31, 2009 is a decade.