[math-fun] When decades start

I’ve heard that, “officially”, the 21st century began on January 1, 2001, not January 1, 2000. Do those officials (whoever they are) also contend that the Aughts (or whatever they’re called) didn’t start until 2001, so that likewise the Teens didn’t start until 2011 and the Twenties won’t start until a year from now? Or do they say that the last year of each century is also the first year of a new decade? Sorry if I’m resurrecting a thread from ten or twenty years ago... Jim Propp