The is no year with number 0.... Before Jan 1st, 1 A.D. came Dec 31, 1 B.C.
So a decade, being 10 years are the years 1 till 10... 10 still being in the first decade or it would be 9 years in stead. And thus 11 started the 2nd decade...
And so, contrary to popular belief, we are now not in the 203rd decade, but in decade 202, and in 2021 the 203rd decade will begin.
Some is true for the 3rd millennium, which did not start in the year 2000, but in 2001...
The term "year 0" is therefore not valid, but often used as a reference to the start of the counting of years, but the "year 0" itself doesn't exist.
