A minor and tangential point: neither the Julian nor the Gregorian calendars (i.e. the two calendars which use BC/BCE and AD/CE) as we have used them from the early middle ages have a year 0, but rather transition from 1BC to 1AD. This may be, as some have argued, because the idea zero hadn't yet come to the West. However, it is worth noting that anno Domini (in the year of the Lord, i.e. "AD") is a construction called "ablative of time within which" and is a form of inclusive counting, so it really means "within the year".