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There are many types of calendars in the world with different systems of measuring time, distinct origins (religious or cultural), and various chronologies (starting events and dates). The Gregorian or Christian calendar (decreed by Pope Gregory XIII in 1582) has become an accepted international standard. This calendar begins with the birth of Christ; the years before Christ (B.C.) increase back in time from the year of Christ's birth. The years since Christ's birth (in 1 B.C.) are "the year(s) of Our Lord" in Latin, anno domini or A.D. and have grown larger in number from one forward. No year zero separates B.C. from A.D. As another example, the Hebrew calendar uses 3761 B.C., which was the year of the creation of the world (according to Old Testament interpretations), as its beginning (or 1 A.M., anno mundi, "the year of the world").

In the past, landmark centuries have also had great significance. When the first millennium was reached in A.D. 1000, in Medieval Europe, various events were predicted, from the return of Christ to a solar eclipse, from a plague to the obliteration of the world. Others chose to forgo doomsday predictions in favor of celebration.

Similarly, as the year 2000 was commonly thought to begin the third millennium, some superstitious persons, numerologists, and others attached unusual significance to a simple tick of the clock. Technically, the new millennium did not begin until January 1, 2001 (because the Christian calendar begins with the year A.D. 1). While this event was marked by many, it did not have the mass appeal of the turning of the year 2000.