I already had this discussion with Shlup already, (very one-sided because I was right and she was so obviously wrong) but a "decade" is just a collection of ten consecutive years. It could be considered associated with every ten years since the beginning of CE/AD, in which case 2010 would not be a new decade (and 2011 would be). However, the term is commonly associated with a group of ten consecutive years so that in XXAX the number for "A" stays the same (e.g., "the '80s," "the '90s," etc.). Neither is wrong, and in the latter sense 2010 would be the start of a new decade. It is arbitrary, but then so is our entire number and calendar system to begin with.