“The year 2009 ends 10 years of years beginning in 2000. It is the last year of the first decade of the 2000s.”
From 0-1 is one year, from 1-2 is two years......from 8-9 is nine years, from 9-10 is ten years. The new decade begins at the beginning of 2010.
If it were money, one half dollar, another half dollar, then you have one dollar. You don’t start off with one dollar, you start of with a fractions of a dollar.
The first decade of the 1000 years beginning with a 2 started in 2000. Yu can explain it your way all you want or you can go with what the world knows and uses.