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There is no year 0. Neither BC nor AD there is a year 1 BC, and a year 1 AD.
Only if you represent the numbers using digits '1',...,'9' only. In that case '1' has the value 0, and '9' the value 8, and your
numbering is 9-based.
Don't mix up digits ans numbers. Digits are for numbers what characters are for words.