Clarity

A decade is a period of ten years. The word is derived from the late Latin decas. Since there is no year 0, I'm going ahead with the idea that a decade goes from year 1 to year 10. Therefore, 2009 is not the end of the first decade of the 21st century. The decade ends on December 31st, 2010. I know ordinal references are not commonly applied to decades (as they are to centuries), but believe it's the more accurate practice. I hope this helps.