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.
Dionysius Exiguus
The epact, since it originally marked the new moon, was zero in all first decennovenal years, making Dionysius the first known medieval Latin writer to use a precursor of the number zero. The Latin word "nulla" meaning "no/none" was used because no Roman numeral for zero existed. To determine the decennovenal year, the Dionysian year plus one was divided by 19. If the result was zero (to be replaced by 19), it was represented by the Latin word "nihil", also meaning "nothing". Both "zeros" continued to be used by (among others) Bede, by whose extension of Dionysius Exiguus’ Easter table to a great Easter cycle all future Julian calendar dates of Easter Sunday were fixed unambiguously at last. However, in medieval Europe one had to wait as late as the second millennium to see the number zero itself come into use, although it had come into being around the year 600 in India.