centuryyear

A century year is a calendar year that is divisible by 100. Such years commonly mark the endpoints of a century in many calendar systems and in year numbering. Examples include the years 1700, 1800, 1900 and 2000.

In the Gregorian calendar, leap years occur every four years, but century years are treated specially: a year that is divisible by 100 is not a leap year unless it is also divisible by 400. Therefore, 1600 and 2000 are leap years, while 1700, 1800 and 1900 are not. This rule keeps the calendar in alignment with the solar year over long periods.

Centuries are named by the span of years they encompass, with century years serving as boundary markers between successive centuries. However, there is variation in how people define the start and end of a century. The astronomical or proleptic Gregorian convention places the 21st century as 2001–2100, whereas some traditional usages treat 1900–1999 as the 20th century.

Historically, century years reflect calendar reforms intended to correct drift from the solar year. They also influence era calculations and digit-level conventions in date notation and programming, where century boundaries often require special handling.

See also: Gregorian calendar, leap year, leap year rules, year.