Year zero
A year zero is a date where the current year for a given calendar system is zero. In systems which include a year zero, this year would be the epoch. Year zero does not exist in the Anno Domini (AD) calendar year system commonly used to number years in the Gregorian calendar and Julian calendar. Instead, AD 1 is treated as the epoch, so that the year 1 BC is followed directly by year AD 1. However, there is a year zero in both the astronomical year numbering system (where it coincides with the Julian year 1 BC), and the ISO 8601:2004 system, a data interchange standard for certain time and calendar information (where year zero coincides with the Gregorian year 1 BC; . There is also a year zero in most Buddhist and Hindu calendars.