A decade is simply a period of 10 years. The word 'decade' itself does not have any inherent positional meaning. Ergo, your understanding of the word is incorrect. The start of a 10-year period is in fact year 0. The 10th year would mark the end of that decade; not the start.
That said, I don’t believe there’s anything more I can add that would change your perspective. So I’ll leave things here