End of 2020 is the end of old decadeĀ 

Dear Sir, — As Sunday’s date was a “palindrome,” 0202 2020, I thought I would write to ask you and your readers why do we now have only nine years in a decade? 
We continually hear of starting “the new decade” in 2020, but the old decade should not finish until the end of 2020! 
Counting started with year one, so the decade ended after year 10 and the next decade started with year 11, and so on, Ad Infinitum.
The “powers that be,” therefore, also got it all wrong with the new Millennium, which was universally celebrated in the start of the year 2000, when the true start was in the beginning of the year 2001. 
Should we alter the calendar? — Yours faithfully,