Well, what I'm saying is that the colloquial definition of a "decade" is different from the determination of the millennium. Both, of course, are arbitrarily defined, but they are arbitrarily defined in distinct ways. This is why we can have a decade that incorporates two different millennia. The second millennium ended on December 31st, 2000, but the "nineties" ended on December 31st, 1999. Thus, the new decade
as generally defined began before the second millennium ended. The years from 2001-2010 were certainly
a decade, being ten years in magnitude. However, if you were to define the "aughts" as the years 2001-2010, you would not only have to deal with where to place the year 2000, but you would also have to exclude 2020 from the "new twenties" and place it in the "new tens", exclude 2030 form the "new thirties" and place it in the "new twenties", etc. As we define a decade, this wouldn't make any sense.
Maybe we should just switch to the Mayan calendar.