For other uses see Calendar (disambiguation). A page from the Hindu calendar 18711872.

A calendar is a system of organizing days for social religious commercial or administrative purposes. This is done by giving names to periods of time typically days weeks months and years. The name given to each day is known as a date. Periods in a calendar (such as years and months) are usually though not necessarily synchronized with the cycle of the sun or the moon. Many civilizations and societies have devised a calendar usually derived from other calendars on which they model their systems suited to their particular needs.

A calendar is also a physical device (often paper). This is the most common usage of the word. Other similar types of calendars can include computerized systems which can be set to remind the user of upcoming events and appointments.

A calendar can also mean a list of planned events such as a court calendar.

The English word calendar is derived from the Latin word kalendae which was the Latin name of the first day of every month.1 Contents 1 Calendar systems 1.1 Solar calendars 1.1.1 Days used by solar calendars 1.1.2 Calendar reform 1.2 Lunar calendars 1.3 Lunisolar calendars 2 Calendar subdivisions 3 Other calendar types 3.1 Arithmetic and astronomical calendars 3.2 Complete and incomplete calendars 4 Uses 5 Currently used calendars 5.1 Fiscal calendars 6 Gregorian calendar with Easter Sunday 7 Physical calendars 8 Legal 9 Calendars in computing 10 See also 10.1 List of calendars 11 Sources 12 Further reading 13 References 14 External links Calendar systems

A full calendar system has a different calendar date for every day. Thus the week cycle is by itself not a full calendar system; neither is a system to name the days within a year without a system for identifying the years.

The simplest calendar system just counts time periods from a reference date. This applies for the Julian day. Virtually the only possible variation is using a different reference date in particular one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of addition and subtraction.

Other calendars have one (or multiple) larger units of time.

Calendars that contain one level of cycles: week and weekday  this system (without year the week number keeps on increasing) is not very common year and ordinal date within the year e.g. the ISO 8601 ordinal date system

Calendars with two levels of cycles: year month and day  most systems including the Gregorian calendar (and its very similar predecessor the Julian calendar) the Islamic calendar and the Hebrew calendar year week and weekday  e.g. the ISO week date Cycles can be synchronized with periodic phenomena: A lunar calendar is synchronized to the motion of the Moon (lunar phases); an example is the Islamic calendar. A solar calendar is based on perceived seasonal changes synchronized to the apparent motion of the Sun; an example is the Persian calendar. A "luni-solar calendar" is based on a combination of both solar and lunar reckonings; examples are the traditional calendar of China the Hindu Calendar in India or the Hebrew calendar. 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. The week cycle is an example of one that is not synchronized to any external phenomenon (although it may have been derived from lunar phases beginning anew every month). Very commonly a calendar includes more than one type of cycle or has both cyclic and acyclic elements. Many calendars incorporate simpler calendars as elements. For example the rules of the Hebrew calendar depend on the seven-day week cycle (a very simple calendar) so the week is one of the cycles of the Hebrew calendar. It is also common to operate two calendars simultaneously usually providing unrelated cycles and the result may also be considered a more complex calendar. For example the Gregorian calendar has no inherent dependence on the seven-day week but in Western society the two are used together and calendar tools indicate both the Gregorian date and the day of week.2 The week cycle is shared by various calendar systems (although the significance of special days such as Friday Saturday and Sunday varies). Systems of leap days usually do not affect the week cycle. The week cycle was not even interrupted when 10 11 12 or 13 dates were skipped when the Julian calendar was replaced by the Gregorian calendar by various countries. Solar calendars Main article: Solar calendar Days used by solar calendars Solar calendars assign a date to each solar day. A day may consist of the period between sunrise and sunset with a following period of night or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year or it may be averaged into a mean solar day. Other types of calendar may also use a solar day. Calendar reform Main article: Calendar reform There have been a number of proposals for reform of the calendar such as the World Calendar International Fixed Calendar and Holocene calendar. The United Nations considered adopting such a reformed calendar for a while in the 1950s but these proposals have lost most of their popularity. Lunar calendars Main article: Lunar calendar Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each lunar phase cycle. Because the length of the lunar month is not an even fraction of the length of the tropical year a purely lunar calendar quickly drifts against the seasons which don't vary much near the equator. It does however stay constant with respect to other phenomena notably tides. An example is the Islamic calendar. Alexander Marshack in a controversial reading3 believed that marks on a bone baton (c. 25000 BC) represented a lunar calendar. Other marked bones may also represent lunar calendars. Similarly Michael Rappenglueck believes that marks on a 15000-year old cave painting represent a lunar calendar.4 Lunisolar calendars A lunisolar calendar is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons. An example is the Hebrew calendar which uses a 19-year cycle. Calendar subdivisions 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 seasons) traditionally used to facilitate the planning of 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 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. Many cultures use different baselines for their calendars' starting years. For example the year in Japan is based on the reign of the current emperor: 2006 was Year 18 of the Emperor Akihito. See Decade Century Millennium Other calendar types Arithmetic and astronomical calendars An astronomical calendar is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the Second Temple. Such a calendar is also referred to as an observation-based calendar. The advantage of such a calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult. An arithmetic calendar is one that is based on a strict set of rules; an example is the current Jewish calendar. Such a calendar is also referred to as a rule-based calendar. The advantage of such a calendar is the ease of calculating when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore even if the calendar is very accurate its accuracy diminishes slowly over time owing to changes in Earth's rotation. This limits the lifetime of an accurate arithmetic calendar to a few thousand years. After then the rules would need to be modified from observations made since the invention of the calendar. Complete and incomplete calendars Calendars may be either complete or incomplete. Complete calendars provide a way of naming each consecutive day while incomplete calendars do not. The early Roman calendar which had no way of designating the days of the winter months other than to lump them together as "winter" is an example of an incomplete calendar while the Gregorian calendar is an example of a complete calendar. Uses The primary practical use of a calendar is to identify days: to be informed about and/or to agree on a future event and to record an event that has happened. Days may be significant for civil religious or social reasons. For example a calendar provides a way to determine which days are religious or civil holidays which days mark the beginning and end of business accounting periods and which days have legal significance such as the day taxes are due or a contract expires. Also a calendar may by identifying a day provide other useful information about the day such as its season. Calendars are also used to help people manage their personal schedules time and activities particularly when individuals have numerous work school and family commitments. People frequently use multiple systems and may keep both a business and family calendar to help prevent them from overcommitting their time. 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 Calendars in widespread use today include the Gregorian calendar which is the de facto international standard and is used almost everywhere in the world for civil purposes including in the People's Republic of China and India (along with the Indian national calendar). Due to the Gregorian calendar's obvious connotations of Western Christianity non-Christians and even some Christians sometimes justify its use by replacing the traditional era notations "AD" and "BC" ("Anno Domini" and "Before Christ") with "CE" and "BCE" ("Common Era" and "Before Common Era"). The Islamic calendar or Muslim calendar or Hijri calendar is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in most of the Muslim countries (concurrently with the Gregorian calendar) and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals. The first year was the year during which the emigration of the Prophet Muhammad from Mecca to Medina known as the Hijra occurred. Each numbered year is designated either H for Hijra or AH for the Latin anno Hegirae (in the year of the Hijra). Being a purely lunar calendar it is not synchronized with the seasons. With an annual drift of 11 or 12 days the seasonal relation is repeated approximately each 33 Islamic years. The Hindu calendars are some of the most ancient calendars of the world. Eastern Christians of eastern Europe and western Asia used for a long time the Julian Calendar that of the old Orthodox church in countries like Russia. For over 1500 years Westerners used the Julian Calendar as well. While the Gregorian calendar is widely used in Israel's business and day-to-day affairs the Hebrew calendar used by Jews worldwide for religious and cultural affairs also influences civil matters in Israel (such as national holidays) and can be used there for business dealings (such as for the dating of checks). The Chinese Hebrew Hindu and Julian calendars are widely used for religious and/or social purposes. The Iranian (Persian) calendar is used in Iran and some parts of Afghanistan. The Ethiopian calendar or Ethiopic calendar is the principal calendar used in Ethiopia and Eritrea. In Thailand where the Thai solar calendar is used the months and days have adopted the western standard although the years are still based on the traditional Buddhist calendar. Bah's worldwide use the Bah' calendar. Even where there is a commonly used calendar such as the Gregorian calendar alternate calendars may also be used such as a fiscal calendar or the astronomical year numbering system.5 Fiscal calendars Main article: Fiscal calendar A fiscal calendar (such as a 4/4/5 calendar) fixes each month at a specific number of weeks to facilitate comparisons from month to month and year to year. January always has exactly 4 weeks (Sunday through Saturday) February has 4 weeks March has 5 weeks etc. Note that this calendar will normally need to add a 53rd week to every 5th or 6th year which might be added to December or might not be depending on how the organization uses those dates. There exists an international standard way to do this (the ISO week). The ISO week starts on a Monday and ends on a Sunday. Week 1 is always the week that contains 4 January in the Gregorian calendar. Fiscal calendars are also used by businesses. This is where the fiscal year is just any set of 12 months. This set of 12 months can start and end at any point on the Gregorian calendar. This is the most common usage of fiscal calendars. Gregorian calendar with Easter Sunday Main article: Gregorian calendar Calculating the calendar of a previous year (for the Gregorian calendar taking account of the week) is a relatively easy matter when Easter Sunday is not included on the calendar. However calculating for Easter Sunday is difficult because the calculation depends on the full moon cycle. Easter Sunday represents the first Sunday after the first full moon after the Vernal Equinox according to the computus. But it is defined as the Sunday following a theoretical Full Moon date falling on or after March 21st and different (though equivalent) calculations are specified by the Papal Bull of 1582 and the British Calendar Act of 1751. So this makes an additional calculation necessary on top of the normal calculation for January 1 and the calculation of whether or not the year is a leap year. There are only 14 different calendars when Easter Sunday is not involved. Each calendar is determined by the day of the week January 1 falls on and whether or not the year is a leap year. However when Easter Sunday is included there are 70 different calendars (two for each date of Easter). Physical calendars A calendar is also a physical device (often paper) (for example a desktop calendar or a wall calendar). In a paper calendar one or two sheets can show a single day a week a month or a year. If a sheet is for a single day it easily shows the date and the weekday. If a sheet is for multiple days it shows a conversion table to convert from weekday to date and back. With a special pointing device or by crossing out past days it may indicate the current date and weekday. This is the most common usage of the word. The sale of physical calendars has been restricted in some countries and given as a monopoly to universities and national academies. Examples include the Prussian Academy of Sciences and the University of Helsinki which had a monopoly on the sale of calendars in Finland until the 1990s. Legal Main article: Docket (court) For lawyers and judges the calendar is the docket used by the court to schedule the order of hearings or trials. A paralegal or court officer may keep track of the cases by using docketing software. Calendars in computing Category:Calendaring standards Electronic calendar See also Calendar reform Calendrical calculation List of international common standards List of unofficial observances by date Real-Time Clock (RTC) which underlies the Calendar software on modern computers. Time for divisions smaller than one day List of calendars Main article: List of calendars Sources Birashk Ahmad (1993) A comparative Calendar of the Iranian Muslim Lunar and Christian Eras for Three Thousand Years Mazda Publishers ISBN 0939214954  Dershowitz Nachum; Reingold Edward M (1997) Calendrical Calculations Cambridge University Press ISBN 0521564743 http://emr.cs.iit.edu/home/reingold/calendar-book/second-edition/  with Online Calculator Zerubavel Eviatar (1985) The Seven Day Circle: The History and Meaning of the Week University of Chicago Press ISBN 0226981657  Doggett LE (1992) "Calendars" in Seidelmann P. Kenneth Explanatory Supplement to the Astronomical Almanac University Science Books ISBN 0935702687 http://astro.nmsu.edu/lhuber/leaphist.html  rni Bjrnsson (1995) 1977 High Days and Holidays in Iceland Reykjavk: Ml og menning ISBN 9979308028 OCLC 186511596  Richards EG (1998) Mapping Time the calendar and its history Oxford University Press ISBN 0198504136  Rose Lynn E (1999) Sun Moon and Sothis Kronos Press ISBN 0917994159  Spier Arthur (1986) The Comprehensive Hebrew Calendar Feldheim Publishers ISBN 0873063988  Dieter Schuh (1973) (in German) Untersuchungen zur Geschichte der Tibetischen Kalenderrechnung Wiesbaden: Franz Steiner Verlag OCLC 1150484  Further reading Fraser Julius Thomas (1987) Time the Familiar Stranger (illustrated ed.) Amherst: Univ of Massachusetts Press ISBN 0870235761 OCLC 15790499  Whitrow Gerald James (2003) What is Time Oxford: Oxford University Press ISBN 0198607814 OCLC 265440481  References New Shorter Oxford English Dictionary Zerubavel The Seven Day Circle (University of Chicago Press 1985). James Elkins Our beautiful dry and distant texts (1998) 63ff. Oldest lunar calendar identified NASA  Year Dating Conventions External links Wikimedia Commons has media related to: Calendars Look up calendar in Wiktionary the free dictionary. Wikisource has the text of the 1911 Encyclopdia Britannica article Calendar. Calendar FAQ Invention of calendar in ancient times an educational web site  "Calendar". Encyclopedia Americana. 1920.  v d eCalendars (list) Wide use Astronomical  Chinese  Gregorian  Hijri  Solar Hijri  ISO Calendar types: Lunar  Lunisolar  Solar Selected use Akan  Armenian   Assyrian  Aztec (Tonalpohualli  Xiuhpohualli)  Babylonian  Bah'  Bengali  Berber  Bikram Samwat  Buddhist  Bulgar   Burmese  Byzantine  Celtic  Coptic Ethiopian  Hebrew  Hellenic  Hindu  Igbo  Inca  Indian  Iranian (Zoroastrian Medieval (Jalali) Modern (Hijri))  Irish  Japanese  Javanese  Juche  Korean  Kurdish  Lithuanian  Malayalam  Maya (Haab'  Tzolk'in)  Minguo  Mongolian  Nanakshahi  Nepal Sambat  Pawukon  Pentecontad  Rapa Nui  Tamil  Thai (Lunar  Solar)  Tibetan  Vietnamese  Xhosa  Yoruba Calendar types: Runic  Mesoamerican (Long Count  Calendar round) Christian variants: Calendar of saints  Eastern Orthodox liturgical calendar  Liturgical year  Revised Julian calendar Rarely used Julian Historical Egyptian  French Republican  Germanic  Roman calendar  Rumi  Soviet  Swedish  Turkmen Martian Darian Alternative Discordian New Age Dreamspell Displays and applications Economic  Perpetual  Wall Proposed calendars The World Calendar  13-month calendar Year numbering Calendar era  Year zero Fictional Middle-earth  Stardate v d eTime Major concepts Time  Eternity  Arguments for eternity  Immortality Deep time  History  Past  Present  Future  Futurology Time Portal Measurement and standards Chronometry  UTC  UT  TAI  Second  Minute  Hour  Sidereal time  Solar time  Time zone Clock  Astrarium  History of timekeeping devices  Horology  Marine chronometer  Sundial  Water clock Calendar  Day  Week  Month  Year  Tropical year  Gregorian  Islamic  Julian Intercalation  Leap second  Leap year Chronology Astronomical chronology  Calendar era  Chronicle  Dating methodologies  Geochronology Geologic Time  Geological history  Periodization  Regnal year  Timeline Religion and mythology Dreamtime  Kla  Kalachakra  Prophecy  Time and fate deities  Wheel of time Philosophy A-series and B-series  B-Theory of time  Causality  Endurantism  Eternal return  Eternalism  Event Perdurantism  Presentism  Temporal finitism  Temporal parts  The Unreality of Time Physical sciences Time in physics  Absolute time and space  Arrow of time  Chronon  Coordinate time Planck epoch  Planck time  Proper time  Spacetime  Theory of relativity Time dilation  Gravitational time dilation  Time domain  T-symmetry Biology Chronobiology  Circadian rhythms Psychology Mental chronometry  Sense of time  Specious present Sociology and anthropology Long Now Foundation  Time discipline  Time use research Economics Time value of money  Time-based currency  Time Banking Related topics Carpe diem  Duration  Hexadecimal time  Metric time  Space  System time  Tempus fugit Time capsule  Time signature  Time travel v d eTime measurement and standards Major subjects Time  Chronometry  Orders of magnitude  Metrology International standards UTC  UTC offset  UT  DUT1  IERS  ISO 31-1  ISO 8601  TAI  12-hour clock  24-hour clock  Barycentric Coordinate Time  Civil time  Daylight saving time  Geocentric Coordinate Time  International Date Line  Leap second  Solar time  Terrestrial Time  Time zone Obsolete standards Barycentric Dynamical Time  Ephemeris time  Greenwich Mean Time  Prime Meridian Time in physics Absolute time and space  Spacetime  Chronon  Continuous time  Coordinate time  Cosmological decade  Discrete time  Planck epoch  Planck time  Proper time  Theory of relativity  Time dilation  Gravitational time dilation  Time domain  T-symmetry Horology Clock  Astrarium  Atomic clock  Complication  Equation of time  History of timekeeping devices  Hourglass  Marine chronometer  Marine sandglass  Radio clock  Sundial  Watch  Water clock Calendar Astronomical  Calculating the day of the week  Dominical letter  Epact  Equinox  Gregorian  Intercalation  Islamic  Julian  Leap year  Lunar  Lunisolar  Seven-day week  Solar  Solstice  Tropical year  Week-day names Archaeology & geology Dating methodologies  Geologic time scale  International Commission on Stratigraphy Astronomical chronology Galactic year  Nuclear time scale  Precession  Sidereal time Units of time Century  Day  Decade  Fortnight  Hour  Jiffy  Lustrum  Millennium  Minute  Month  Saeculum  Second  Shake  Tide  Week  Year Related topics Chronology  Duration  Mental chronometry  Metric time  System time  Time value of money  Timekeeper v d eChronology Main articles Time  Astronomy  Geology  Paleontology  Archaeology  History Eras and epochs Calendar eras Ab urbe condita  Anno Domini (Common Era)  Anno Mundi  Byzantine era  Spanish era  Before Present  Hijri  Egyptian  Sothic cycle  Hindu units of measurement  Hindu Yugas Regnal year Canon of Kings  Lists of kings  Limmu  Seleucid era Era names Chinese  Japanese  Korean  Vietnamese Calendars (Pre-)Julian Pre-Julian Roman  Original Julian  Proleptic Julian  Revised Julian Gregorian Gregorian  Proleptic Gregorian  Old Style and New Style dates Astronomical Lunisolar  Solar  Lunar  Astronomical year numbering Others Iranian  Islamic  Chinese sexagenary cycle  ISO week date Astronomic time   Chronology Portal Cosmic Calendar  Ephemeris  Galactic year  Metonic cycle  Milankovitch cycles Geologic time Concepts Deep time  Geological history of Earth  Geological time units Standards Global Standard Stratigraphic Age (GSSA)  Global Boundary Stratotype Section and Point (GSSP) Methods Chronostratigraphy  Geochronology  Isotope geochemistry  Law of superposition  Optical dating  Samarium-neodymium dating Archaeological methods Absolute dating Incremental dating  Archaeomagnetic dating  Dendrochronology  Glottochronology  Ice core  Lichenometry  Paleomagnetism  Radiocarbon dating  Radiometric dating  Tephrochronology  Thermoluminescence dating  Uranium-lead dating Relative dating Seriation  Stratification  Fluorine absorption dating Genetic methods Amino acid dating  Molecular clock Related topics Chronicle  New Chronology  Periodization  Synchronoptic view  Timeline  Year zero  Circa  Floruit