It depends which calendar you use! Every calendar picks a basically arbitrary system to uniquely identify each year, and in some of them “year 0” doesn’t refer to any year.
The Gregorian, for example, goes directly from 1 BC to 1 AD, since 1 BC is “the first year before Christ” and 1 AD is “the first in the years of our lord.” This doesn’t make much mathematical sense, but it’s not like there was a year that didn’t happen–they just called one year 1 BC, and the next year 1 AD.
ISO 8601 is based on the Gregorian calendar, but it includes a year 0. 1 BC is the same year as +0000; thus 2 BC is -0001, and all earlier years are likewise offset by 1 between the two calendars.