The key here, as @stranger_in_a_strange_land points out, is that there was no year zero. A decade is ten years. The first decade A.D. was 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, because there was no zero. So the second decade was 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and this decade is 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010. The new decade does not begin until January 1, 2011. Otherwise some decade somewhere has to get short changed by a year. Now you can describe the decade that began January 1, 2000 and ends December 31, 2009, and it is indeed a decade, but it is not the 202nd decade, because it has no relationship to earlier decades. I could just as well call the ten years from my 1st birthday to my 11th a decade, but it’s not meaningful to call it the decade.