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.
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Although the time code only includes two digits of year, it is possible to deduce two bits of century using the day of week. There is still a 400-year ambiguity, as the Gregorian calendar repeats weeks every 400 years, but this is sufficient to determine which years ending in 00 are leap years.