• Scheduling

Calendar

A calendar is a system for assigning calendar dates to days. The dates may be based on the perceived motion of astronomical objects. A calendar can also be a physical device (often paper) that illustrates the system (for example, a desktop calendar). The term is also used to indicate a particular set of planned events (for example, court calendar).

Table of contents

1 Calendar systems 1.1 Solar calendars 1.1.1 Days used by solar calendars 1.1.2 Julian and Gregorian calendars 1.1.3 Future reform 1.2 Lunar calendars 1.3 Fiscal calendars 2 Calendar subdivisions 3 Other calendar types 3.4 Complete and incomplete calendars 3.5 Pragmatic, theoretical and mixed Calendars 4 Uses 5 Currently used calendars 6 List of calendars 6.6 In current use 6.7 Obsolete 6.8 Proposed 7 See also 8 References

Calendar systems

A lunar calendar is synchronized to the motion of the Moon (moon phases); an example is the Islamic calendar.

A solar calendar is based on perceived seasonal changes synchronized to the motion of the Sun; an example is the Persian calendar.

A lunisolar calendar is synchronized to the motions of both the Moon and the Sun; an example is the Jewish calendar.

An arbitrary calendar is not synchronized to either the Moon or the Sun; examples are the week and the Julian day used by astronomers.

There are some calendars that appear to be synchronized to the motion of Venus, such as some of the ancient Egyptian calendars; synchronization to Venus appears to occur primarily in civilizations near the Equator.

Solar calendars

Julian and Gregorian calendars

Unfortunately, Earth's tropical year is a little less 365.25 days (it is approximately 365.242 days), so this calendar, too, slowly drifted out of sync with the seasons. For such reasons, the Gregorian calendar was later adopted by most of the West, starting in 1582, and it has since become the world's dominant civic calendar.

Future reform

Lunar calendars

Fiscal calendars

Calendar subdivisions

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 done with leap years. 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. 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.

Other calendar types

Pragmatic, theoretical and mixed Calendars

Uses

Calendars are also used as part of a complete timekeeping system: date and time of day together specify a moment in time. In the modern world, written calendars are no longer an essential part of such systems, as the advent of accurate clocks has made it possible to record time independently of astronomical events.

Currently used calendars

Even where there is a commonly used calendar such as the Gregorian calendar, alternate calendars may also be used, such as a fiscal calendar.

List of calendars

In current use

• Gregorian calendar
• Julian calendar
• Chinese calendar
• Japanese calendar
• Hebrew calendar
• Iranian calendar
• Irish calendar
• Islamic calendar
• Hindu calendars
• Revised Julian calendar
• Coptic calendar
• Thai solar calendar
• Zoroastrian calendar (including Parsi)
• Malayalam calendar
• Bahai calendar
• Fiscal year
• Julian date
• ISO Week-Date calendar
• 53-week calendar
• Metonic calendar

• Aztec calendar
• Babylonian calendar
• Egyptian calendar
• French Revolutionary calendar
• Hellenic calendar
• Maya calendar
• Roman calendar
• Positivist calendar
• Soviet revolutionary calendar
• Thai lunar calendar

• Darian calendar for use on Mars, the four Galilean moons of Jupiter, and Titan.
• calendar reforms:
  • World calendar
  • International Fixed Calendar (also called the International Perpetual calendar)

• liturgical year
• calendar of saints
• Christian calendar
• Calculating the Day of the Week
• Cycles
• Calendar Girls - 2003 movie starring Helen Mirren and Julie Walters

• Calendrical Calculations; Nachum Dershowitz and Edward M. Reingold; Cambridge University Press, 1997; ISBN 0-521-56474-3
• A comparative Calendar of the Iranian, Muslim Lunar,and Christian Eras for Three Thousand Years; Ahmad Birashk; Mazda Publishers, 1993; ISBN 0-939214-95-4
• The Comprehensive Hebrew Calendar; Arthur Spier; Feldheim Publishers, 1986; ISBN 0-87306-398-8
• High Days and Holidays in Iceland; Árni Björnsson; Mál og menning, 1995; ISBN 9979-3-0802-8
• Explanatory Supplement to the Astronomical Almanac; P. Kenneth Seidelmann, ed.; University Science Books, 1992; ISBN 0-935702-68-7; Chapter 12: Calendars by L. E. Doggett
• Sun, Moon, and Sothis; Lynn E. Rose; Kronos Press, 1999; ISBN 0-917994-15-9
• Untersuchungen zur Geschichte der Tibetischen Kalenderrechnung; Dieter Schuh; Franz Steiner Verlag GMBH, 1973
• Frequently Asked Questions about Calendars: http://www.tondering.dk/claus/calendar.html
• DMOZ Calendars and Timekeeping category: http://dmoz.org/Science/Astronomy/Calendars_and_Timekeeping/
• Worldwide Calendar Directory
• 400 profiled categorised "Calendars" sites on Zeal

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