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 1900 problem
The most common issue raised by the year 1900 problem regards people's ages. Often, a person's birth year was registered with only two digits, on the assumption that either it was not important exactly how old a person is, or that no one lives longer than one hundred years. In several countries, especially in Europe, a national identification number was introduced (often in the 1950s), including two-digit information about the birth year.