Kilopascal wrote in USMA 10288 >There is some debate over the propriety of a year 0. The program will >convert Gregorian dates in year 0, because its basis is a proleptic >calendar. It will also convert dates prior to year 0 specified in negative >numbers. So year 0 corresponds to year 1BC in history; year -1 corresponds >to year 2BC in history. I believe that this notation used used by astronomers. It simplifies calculating the interval of time between a BC event and an AD event. Thus there are 2 years between -1 BC and 1 AD. By the usual convention there is only 1 year between the 2 dates.
