Year zero
Historians have never included a year zero. This means that between, for example, and , there are 999 years: 500 years BC, and 499 years AD preceding 500. In common usage "anno Domini" 1 is preceded by the year 1 BC, without an intervening year zero. Neither the choice of calendar system (whether Julian or Gregorian) nor the era ("Anno Domini" or Common Era) determines whether a year zero will be used. If writers do not use the convention of their group (historians or astronomers), they must explicitly state whether they include a year 0 in their count of years, otherwise their historical dates will be misunderstood.
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.