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.
Year Zero (political notion)
The term Year Zero ( "chhnam saun"), applied to the takeover of Cambodia in April 1975 by the Khmer Rouge, is an analogy to the Year One of the French Revolutionary Calendar. During the French Revolution, after the abolition of the French monarchy (September 20, 1792), the National Convention instituted a new calendar and declared the beginning of the Year I. The Khmer Rouge takeover of Phnom Penh was rapidly followed by a series of drastic revolutionary de-industrialization policies resulting in a death toll that vastly exceeded that of the French Reign of Terror.