Bizarre.
So a year pronounced "nineteen eighty" is not part of the eighties, but a year pronounced "nineteen ninety" is?
From a cultural standpoint, the eighties were Reagan’s decade, so his inauguration day was the key date.
The first decade of the 21st century began at 8:46am on September 11, 2001. The attacks have defined the decade.
From a purely math standpoint, the next decade begins on the first day of the year 2011, but from a cultural standpoint, the decade really begins on a critical moment that will end up defining the decade.