There was no zero.
At the end of year 1, there had been one year.
At the end of year 10, there had been one decade.
At the end of year 100, there had been one century.
At the end of year 1000, there had been one millenium.
It goes backwards the same way. The first century BC includes the years 1BC to 100BC, the second century BC goes from 101BC to 200BC, and so on. There's a zero point between 1BC and 1AD, but since there's no zero year, new decades, centuries, etc, will always start on a year ending with 1.
Interestingly, if there were a year zero, BC would still have the same problem. AD would go 0-9, 10-19, etc., but BC would still go from 1BC-10BC, 11BC-120BC, etc. To make it work in a way that marks new centuries with '00 years, we would need two year zeros, 0BC and 0AD.
This is done (though not with years) in
some numbering systems.