It’s like the 21st Century didn’t start until 2001. I remember discussions leading up to 2000 on that.
But that's an ordinal definition of century: first century, eleventh century, eighteenth century, etc.
As far as informal, colloquial definitions of cardinal decades, it is ridiculous to say the eighties did not start until 1981.
Aha. So now we know why the dreaded Y2K collapse never happened when 2000 AD started!