[I want the USA back]

It begins wherever you place the starting point, and ends 10 years later.

[Dilbert San Diego]

I know that technically, the decade starts Jan. 1, 2021.

And this is because there was no year zero; our calendar as currently structured counts years from 1 A.D. So the first decade was the year 1 to the year 10. Meaning that years ending in zero will be the end of any decade.

But in popular usage, the decade begins in the years ending in zero and end in the years ending in nine. As we see now, years have switched from being something “teen” so something “twenty”. So in popular usage, we are now in the decade of the twenties.

Can both be right? A decade can be any ten year period.

[House Atreides]

It begins wherever you place the starting point, and ends 10 years later.
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It ends at the end of the day before “10 years later” House says as he joins the quibbling in the thread. ;-)