2010 ends the last decade, which started in 2001.
2011 will start the next decade, which ends in 2020... and so on.
You start counting with 1, not 0. Count to 10; the next lot of 10s starts with 11.

But not many people give tuppence for for all that; they celebrate the millenium start at 2000, when it is in fact the last year of the 20th century. The 21st century started with 2001.

And while we're on about the illogical use of numbers ... what about those damn small billions the Americans use (which our beloved Government in GB has also saddled us with) - their billion is only a thousand million, not the real billion which is a million million. Trillion = million million million (three lots, tri = three); the US trillion is 1,000,000,000,000 (thousand thousand million) i.e. our British billion.

Maybe no-one else cares, but it gets right up my nose.