Calendar

==Subdivisions==
{{see also|Decade|Century|Millennium}}
[[File:Perpetual calendar in Catalan circa year 1925 Patent in Barcelona.jpg|left|thumb|Catalan early 20th century perpetual calendar]]
Nearly all calendar systems group consecutive days into "months" and also into "years". In a ''solar calendar'' a ''year'' approximates Earth's tropical year (that is, the time it takes for a complete cycle of [[season]]s), traditionally used to facilitate the planning of [[agriculture|agricultural]] activities. In a ''lunar calendar'', the ''month'' approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week.

Because the number of days in the ''tropical year'' is not a whole number, a solar calendar must have a different number of days in different years. This may be handled, for example, by adding an extra day in [[leap year]]s. The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as [[Intercalation (timekeeping)|intercalation]]. Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.

Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years. Many cultures use different baselines for their calendars' starting years. Historically, several countries have based their calendars on [[regnal years]], a calendar based on the reign of their current sovereign. For example, the year 2006 in [[Japan]] is year 18 Heisei, with Heisei being the era name of [[Emperor of Japan|Emperor]] [[Akihito]].