depends wich year 0 you're refering to. You have the chinese, the christian, the muslim, the north korean (my personal favorite) and many more.
Hi, not discouraging more discussion here, but FYI, you'll find some good info on the development of the Gregorian calendar, plus several examples of other/older dating systems in the FAQ:
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This comment from the FAQ is probably the best.
One thing is doesn't mention is that there actually wasn't a year 0. The AD/CE system doesn't include a year 0. It goes from 1 BC to AD 1. That throws off BC calculations by a year, but that only really matters for events near the calendar's flip over.