I’m no expert but I assume that the year Christ died would be “year zero” (assuming you’re talking about anno Domini (AD) and before Christ (BC)) since we started counting after that.

EDIT: reading more on the topic I might be completely incorrect with my above statement. If someone else knows, please do correct me

EDIT 2: I found this on Wikipedia which talks about a “year zero”

When you consider the time as a number line, years are not points at integers (which would in some way warrant a year 0), but rather periods between them. Year 1 is the period between 0 and 1, and before that was -1 to 0, or year -1. There is no year 0, because there isn’t anything between 0 and 0