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 1900 problem
When handling the year 2000 problem, measures were sometimes taken to avoid or rectify this: modifying the national identification number, for instance. For example, year is recorded only with two digits in Bulgarian Uniform civil number; however, a solution was ready as early as the inception of the system in 1975: 20 was added to the month number for individual born before 1900 and 40 for those born in or after 2000.