Year zero

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A year zero does not exist in the Anno Domini (AD) calendar year system commonly used to number years in the Gregorian calendar (nor in its predecessor, the Julian calendar); in this system, the year 1 BC is followed directly by year AD 1 (which is the year of the epoch of the era). 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; see conversion table). There is also a year zero in most Buddhist and Hindu calendars.