There was no year zero.

And there really wasn’t a year 1 AD through about year 524 AD that people living at those times kept track of. Those years were numbered after the fact by Dionysius in 525 AD. People living at during those times before that used various other ways of counting years (if they did at all), such as Roman emperor reigns.

So nobody living in, say, what we now call 300 AD would have ever said “the year right now is 300 AD.”