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Why are you counting from 0? What do you mean by not authoriative in this context? This is just how the Gregorian calendar works. https://en.wikipedia.org/wiki/Anno_Domini#No_year_zero:_star...
Why are you counting from AD 1, which nobody alive at the time called AD 1? It's natural to align the decades, centuries, an millennia with a decimal radix. You've got the year digit, the decade digit, the century digit, and the millennium digit. Decades are always talked about this way, e.g. "the 80's", and I'd wager that most people think about centuries and millenniums this way. The fact that the year number is de…
I agree about your 80s example (of course I use it in the same way), but that doesn't change the fact that it ultimately doesn't make sense because you'll be off by one when counting backwards. So for colloquial conversations it's fine, for serious date calculations, you cannot just have one "decade" with only 9 years.