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.
Millennium
The "year 2000" has also been a popular phrase referring to an often utopian future, or a year when stories in such a future were set, adding to its cultural significance. There was also media and public interest in the Y2K bug. People liked to compared their odometer as a reason to celebrate the new millennium which goes from 1999 to 2000. Thus, the populist argument was that the new millennium should begin when the zeroes "rolled over" to 2000, i.e., the day after December 31, 1999. Some people thought that the change of the hundreds digit in the year number, and the zeros rolling over, created a sense that a new century had begun.