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.
Library of Congress Control Number
In its most elementary form the number includes a year and a serial number. The year has two digits for 1898 to 2000, and four digits beginning in 2001. The three ambiguous years (1898, 1899, and 1900) are distinguished by the size of the serial number. There are also some peculiarities in numbers beginning with a "7" because of an unsuccessful experiment applied between 1969 and 1972.