There is no year 0 in the Gregorian calendar. The years go from 1 B.C. to 1 A.D.
the years of the first decade A.D. counted off as years 1,2,3,4,5,6,7,8,9,10
For this reason, the real end of the decade is December 31, 2010.
It's because there is no year 0. Do the math, people.