So, a decade has ten years, counted as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. Seems to end with a zero, eh.
Unless the CE started with a year zero instead of a year one, then decades, centuries and millenniums end with the end of the tenth year in that sequence.
But further discussion of this technicality is just as nugatory as the discussion of noon as am or pm.