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.
1000
Year 1000 (M) was a leap year starting on Monday (link will display the full calendar) of the Julian calendar. In the proleptic Gregorian calendar, it was a non-leap century year starting on Wednesday (like 1800). It was also the last year of the 10th century as well as the last year of the 1st millennium of the Dionysian era ending on December 31st, but the first year of the 1000s decade.