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.
Names for the number 0 in English
It is quite common in English to describe decades with a plural of the first year of the decade for example "the nineties" which can be written as "the 90s". There was a debate on what to call the first decade of the 21st century, as "00s" is not usually pronounced as a word. Apart from side stepping the issue and calling the decade the "two-thousands" (2000s) there is no agreed wording, although in Britain and some other countries "noughties" was also used.