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.
Upside down year
An upside down year is a year which reads the same when the 'year number' is turned upside down, when the numerals which form the year's number appear the same when rotated 180 degrees (turned upside down).