Year zero
Year zero does not exist in the anno Domini system usually used to number years in the Gregorian calendar and in its predecessor, the Julian calendar. In this system, the year is followed by . However, there is a year zero in astronomical year numbering (where it coincides with the Julian year ) and in ISO 8601:2004 (where it coincides with the Gregorian year ) as well as in all Buddhist and Hindu calendars.
Year zero
In astronomy, for the year AD 1 and later it is common to assign the same numbers as the Anno Domini notation, which in turn is numerically equivalent to the Common Era notation. But the discontinuity between 1 AD and 1 BC makes it cumbersome to compare ancient and modern dates. So the year before 1 AD is designated 0, the year before 0 is −1, and so on.