You are reiterating my point. I may have failed to convey the irony in my
earlier reply. The first decade of our current calendar may have ended on
the first day of year 11, but a decade per se has an arbitrary start point.
I'd even venture to say that it need not begin at the start of a calendar
year.
One popular dictionary's entry:
1) a period of ten years: the three decades from 1776 to 1806.
2) a period of ten years beginning with a year whose last digit is zero:
the decade of the 1980s.
3) a group, set, or series of ten.