Interesting that there seem to be a large number of scholars who would accept either 4 AD or 8 AD was the first AD leap year. But less clear what such a year would have been called by Romans at the time. I'm assuming it wasn't common to include full dates (including the year #) on written documents at the time, otherwise surely we'd know pretty much exactly. I gather AD-based year numbering wasn't actually introduced until ~525 AD (but before that a number of systems had been in use).

0 is a year if we say it is, which astronomers have for hundreds of years (because its extremely mathematically convenient), and which ISO-8601 does.

Year 0 is what historians like to call 1 BC, but historians are also just making that up. Nobody numbered years from Anno Domini anywhere near year 0.

I'm sure it's clear to everybody here why year 0, -1, and -2 are significantly better arbitrary, ahistorical names for the three years preceding year 1.