It would have been if there had been a year 0, but there was no year 0.
So what decade do you put day 12-31-00 into? It has to be considered the last day of the first year of the first decade of the first century. (Do not confuse my argument with the years BC and the years AD). I am making the arguemnt as if the beginning of time began on 1-1-00.
There is considerable disagreement on your point.
From Wiki: "Some writers like to point out that since the common calendar starts from the year 1, its first full decade contained the years from 1 to 10, the second decade from 11 to 20, and so on. The interval from the year 2001 to 2010 could thus be called the 201st decade, using ordinal numbers. However, contrary to practices in referencing centuries, ordinal references to decades are quite uncommon."
From the Cambridge online dictionary: "a period of ten years, especially a period such as 1860 to 1869, or 1990 to 1999"
A decade can be broken down in two ways...and in common usage...its 0-9. That's 10 years. You're straining on a gnat...