Sorry Stilltortoise, Peter is correct.
There was no year Zero.
The calendar goes, start of time…., IV BC, III BC, II BC, I BC, I AD, II AD, III AD,… MMX AD.
Both the Julian and Gregorian calendars where based on Roman numerals.
Otherwise we would only be in the 20th Century as if follows that the
1st 100 years (0-99) would have been the 0 Century.