You start counting from 1 not zero
1,2,3,4,5,6,7,8,9,10
not 0,1,2,3,4,5,6,7,8,9
2010 is the 10th year of 2000
2011 is the first year of 2010
2020 is the 10th year of 2010
201(1) = 1
201(2) = 2
201(3) = 3
the decade is complete at the end of 2020
2021 you start count at 1
By this logic the year 2000 is part of the 90's and the last year of the 20th century



