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calendar [Lat., from Kalends], system of reckoning time for the practical purpose of recording past events and calculating dates for future plans. The calendar is based on noting ordinary and easily observable natural events, the cycle of the sun through the seasons with equinox and solstice, and the recurrent phases of the moon.

Measures of Time

The earth completes its orbit about the sun in 365 days 5 hr 48 min 46 sec—the length of the solar year. The moon passes through its phases in about 291/2 days; therefore, 12 lunar months (called a lunar year) amount to more than 354 days 8 hr 48 min. The discrepancy between the years is inescapable, and one of the major problems since early days has been to reconcile and harmonize solar and lunar reckonings. Some peoples have simply recorded time by the lunar cycle, but, as skill in calculation developed, the prevailing calculations generally came to depend upon a combination.

The fact that months and years cannot be divided exactly by days and that the years cannot be easily divided into months has led to the device of intercalation (i.e., the insertion of extra days or months into a calendar to make it more accurate). The simplest form of this is shown in ancient calendars which have series of months alternating between 30 and 29 days, thus arriving at mean months of 291/2 days each. Similarly four years of about 3651/4 days each can be approximated by taking three years of 365 days and a fourth year of 366. This fourth year with its intercalary day is the leap year. If calculations are by the lunar cycle, the surplus of the solar over the lunar year (365 over 354) can be somewhat rectified by adding an intercalary month of 33 days every three years.

Reckoning of day and year was considered necessary by many ancient peoples to determine sacred days, to arrange plans for the future, and to keep some intelligible record of the past. There were, therefore, various efforts to reconcile the count in solar, lunar, and semilunar calendars, from the Egyptians and the Greeks to the Chinese and the Maya. The prevailing modern method of constructing a calendar in the Christian West came originally from the Egyptians, who worked out a formula for the solar year (12 months of 30 days each, five extra days a year, and an extra day every four years) that was to be adopted later by the Romans.

Development of the Modern Calendar

The Early Roman Calendar

In its most primitive form the Roman calendar apparently had 10 months, which were (to use corresponding English terms whenever possible): March (31 days), April (29 days), May (31 days), June (29 days), Quintilis (31 days), Sextilis (29 days), September (29 days), October (31 days), November (29 days), and December (29 days). To fill out the 365 days a number of blank days or occasional intercalary months were used. Later, January (29 days) and February (28 days) were added at the end of the year.

In the time of the early republic the so-called year of Numa was added. The Romans thus arrived at a cycle of four years: the first year and the third year had four months of 31 days, seven of 29, and one, February, of 28; the second year had a February of 23 days and an intercalary month of 27 days; the fourth year had a February of 24 days and an intercalary month. The chief trouble with this system was that in a four-year cycle there were four days too many. What was worse, the pontifex maximus was given the power soon after 200 B.C. to regulate the calendar, and the practice grew of using the intercalations for the promotion of political ends to lengthen or to shorten an official's term.

The Julian Calendar

When Julius Caesar became pontifex maximus, the Roman calendar had been so much abused that January was falling in autumn. At this point the methods of the Egyptian calendar were borrowed for the Roman. Julius Caesar, on the advice of the astronomer Sosigenes, added 90 days to the year 46 B.C. (67 days between November and December, 23 at the end of February). This caused the spring of 45 B.C. to begin in March. To retain this position of the seasons, he changed the length of most of the months: March, May, Quintilis (later named July after Julius Caesar), and October he left as they were; he added 2 days each to January and Sextilis (later named August to honor the Emperor Augustus); February was 28 days long except that in every fourth year a day was inserted between the 23d and the 24th of the month.

In Roman computation three days in the month were used for counting the date. These three were the Kalends (1st day of the month), the Nones (the 7th day in March, May, July, and October, the 5th in the other months), and the Ides (the 15th day in March, May, July, and October, the 13th in the other months). The days were counted before, not after, the Kalends, Nones, and Ides. Thus, Jan. 10 was the fourth day before the Ides of January or the fourth day of the Ides of January, because the Romans counted inclusively. Jan. 25 was the eighth of the Kalends of February, Feb. 3 was the third of the Nones of February. Feb. 23 was the seventh of the Kalends of March and remained so when an intercalary day was inserted every fourth year between it and Feb. 24; hence in a leap year there were two days counted as the sixth of the Kalends of March. The leap year was therefore called bissextile [Lat.,=sixth twice]. There is a legend that alterations in the length of the months were made later by Augustus to flatter his own vanity, but there seems to be no foundation for this story.

The Gregorian Calendar

The Julian year is 365 days 6 hr, hence a little too long. Therefore, by the 16th cent. the accumulation of surplus time had displaced the vernal equinox to Mar. 11 from Mar. 21, the date set in the 4th cent. In 1582 Pope Gregory XIII rectified this error. He suppressed 10 days in the year 1582 and ordained that thereafter the years ending in hundreds should not be leap years unless they were divisible by 400. The year 1600 was a leap year under both systems, but 1700, 1800, and 1900 were leap years only in the unreformed calendar. The reform was accepted, immediately in most Roman Catholic countries, more gradually in Protestant countries, and in the Eastern Church the Julian calendar was retained into the 20th cent. The present generally accepted calendar is therefore called Gregorian, though it is only a slight modification of the Julian.

The reform was not accepted in England and the British colonies in America until 1752. By that date the English calendar was 11 days different from that of continental Europe. For the intervening period before the reform was introduced into the English calendar, the Gregorian style is called the New Style (N.S.), and the Julian the Old Style (O.S.). New Style years begin Jan. 1, but Old Style years began usually Mar. 25. Thus Washington's birthday, which is Feb. 22, 1732 (N.S.), was Feb. 11, 1731 (O.S.). To avoid confusion sometimes both styles are given; thus 11 Feb. 1731/22 Feb. 1732.

The Christian Ecclesiastical Calendar

The church calendar with its movable feasts shows an interesting example of a harmony of several different systems. The key is the reconciliation of the seven-day week with the Roman calendar (see week). The resurrection of Jesus has always been traditionally reckoned as having taken place on a Sunday (first day of the week); hence the annual feast celebrating the event, Easter, should fall on a Sunday. The Bible places the Passion with relation to the Passover. Since the Jewish Passover is on the evening of the 14th (eve of the 15th) Nisan (see below), it may fall on any day of the week; hence Easter must fall on a Sunday near the 14th Nisan. In ancient times some Eastern Christians celebrated Easter on the 14th Nisan itself; these were called Quartodecimans [Lat.,=fourteenth]. In 325 the First Council of Nicaea determined that Easter should fall on the Sunday following the next full moon after the vernal equinox, the full moon being theoretically the 14th day, and Nisan beginning with a new moon in March. The vernal equinox was considered by the church to fall on Mar. 21. The paschal, or Easter, moon is the full moon, the 14th day of which falls after (but not on) Mar. 21.

Today Easter is calculated according to a system that does not take all factors of the lunar period into consideration, and it nearly always varies somewhat from what it should be according to true astronomical calculation. Several different systems have been used for determining Easter. In the 6th and 7th cent. in England, there was a great dispute between Christians who derived their rite from the Celts and Christians who had been converted as a result of the mission of St. Augustine. The dispute was settled at the Synod of Whitby in favor of the Roman system, which prevailed from that time over the entire West. For a conventional means of computing Easter, see the Anglican Book of Common Prayer.

The Jewish Calendar

The Jewish calendar is today a lunisolar or semilunar calendar, i.e., an adjustment of a lunar calendar to the solar year. The months are Tishri (30), Heshvan—sometimes also called Marheshvan—(29 or 30), Kislev (29 or 30), Tebet (29), Sebat or Shebat (30), Adar (29), Nisan (30), Iyar (29), Sivan (30), Tammuz (29), Ab (30), and Elul (29). The intercalary month of 30 days, Adar II, is added after Adar, Nisan being in ancient times the first month. The intercalation is arranged to take place seven times in 19 years; this is called the Metonic cycle after the Greek astronomer Meton who proposed it about 432 B.C. to express the relation between a lunar and solar year. The common year is referred to as a defective, regular, or perfect year, depending upon whether its length is 353, 354, or 355 days; the leap year may have 383 (defective), 384 (regular), or 385 (perfect) days. The Jewish civil year begins about the autumnal equinox, with the festival of Rosh ha-Shanah (the first of Tishri), which in 1999 fell on Sept. 11, marking the start of the Jewish year 5760.

The Islamic Calendar

The Islamic calendar is the only widely used purely lunar calendar, its year varying from 354 to 355 days. Hence the seasons and months have no connection, and there are about 33 years to every 32 Gregorian years. The months are Muharram (30), Safar (29), 1st Rabia (30), 2d Rabia (29), 1st Jumada (30), 2d Jumada (29), Rajab (30), Shaban (29), Ramadan (the fast, 30), Shawwal (29), Dhu-l-Kada (30), and Dhu-l-Hijja (month of the pilgrimage, 29 or 30). The first day of the Islamic calendar, Muharram 1, A.H. 1, was July 16, 622, in the Western calendar (A.H. [Anno Hegirae=in the year of the Hegira] is used to indicate the Islamic year). Muharram 1, A.H. 1420 was Apr. 17, 1999.

Other Calendars

The old Chinese calendar was devised to have six 60-day cycles, each cycle having 10-day periods and three such periods going to make up a month. By the 5th cent. B.C. the solar year was calculated at 365.2444 solar days and the solar month at 29.53059 days. The difference between solar time and the cycles was adjusted by intercalary months and shorter intercalary periods. The years were arranged in major cycles of 60 years with minor cycles of 5 years each. An interesting calendar is that of the Maya, who used a year of 365 days divided into 18 20-day periods, with a 5-day period at the end. A cycle of 260 days was used to name days. These two recurrent cycles resulted in a great cycle of 52 years. This calendar was carefully calibrated, but the year was never readjusted to the error in its length; instead, the feasts and dates were adjusted to the calendar. The Aztec calendar was very similar. Many attempts have been made to devise new calendars, adjusting the months more regularly to the solar year, discarding the week, making the months equal in length, and the like, but they have never been widely adopted. The most celebrated is the French Revolutionary calendar.

Reckoning the Dates Assigned to Years

The Athenian system of identifying years by archons, the Roman system of identifying them by consuls, and the system of reckoning by the year of the reign of a given king or other ruler offer enormous difficulties, and the establishment of chronology is one of the major problems in ancient and medieval history. (The classic work on chronology is that of the Benedictines, first published in 1750, L'Art de vérifier les dates des faits historiques [the art of verifying the dates of historical acts].) For the method of computing years from a fixed point (e.g., the birth of Jesus and the Hegira), see era. The adoption of such era systems has made computation of time much easier.


See P. W. Wilson, The Romance of the Calendar (1937); H. Watkins, Time Counts: The Story of the Calendar (1954); K. G. Irwin, The Three Hundred Sixty-Five Days (1963); J. E. S. Thompson, Mayan Hieroglyphic Writing (3d ed. 1971); F. Parise, ed., The Book of Calendars (1982).

The word Calendar consist of two words: 1)Cal(in Pashto means Year, in Hindi and Persian is Sal- also means Year).In Pashto question : so kalen ye? means how old are you? 2)Dar (Pashto - means having, Owning and in Persian - in). So we can say that Calendar means Having or owning or in the Year.

A calendar is a system of organizing days for a social, religious, commercial or administrative purpose. This organization 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 cycles of some astronomical phenomenon, such as 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.

As a subset, calendar is also used to denote a list of particular set of planned events (for example, 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.

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:

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.
  • 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. A lunisolar calendar is synchronized both to the motion of the moon and to the apparent motion of the sun; an example is the Hebrew calendar.

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.

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

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

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

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.

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.

Lunar calendars are believed to be the oldest calendars invented by mankind. Cro-Magnon people are claimed to have invented one around 32,000 BC.

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 (29 February) 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.


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 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 also.

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 Persian calendar is used in Iran and Afghanistan. The Islamic calendar is used by most non-Persian Muslims worldwide. The Chinese, Hebrew, Hindu, and Julian calendars are widely used for religious and/or social purposes. 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.

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.

Fiscal calendars

A fiscal calendar (such as a 5/4/4 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 5 weeks (Sunday through Saturday), February has 4 weeks, March has 4 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.

Gregorian calendar with Easter Sunday

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 requires the knowledge of the full moon cycle. Easter Sunday is on the first Sunday after the first full moon after the Vernal Equinox according to the computus. So, this makes an additional calculation necessary on top of the normal calculation for January 1st 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 1st 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.


For lawyers and judges, the calendar is the docket used by the court to schedule the order of hearings or trials. This is especially used in a criminal calendar. A paralegal or court officer may actually keep track of the cases on the calendar or docket, by use of docketing software or law practice management software.

Calendars in computing


There are different layouts for calendars.

See also

List of calendars


  • 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
  • Calendrical Calculations; Nachum Dershowitz and Edward M. Reingold; Cambridge University Press, 1997; ISBN 0-521-56474-3; Book Info; Online Calculator
  • Eviatar Zerubavel, The Seven Day Circle: The History and Meaning of the Week. University of Chicago Press, 1985. ISBN 0-226-98165-7
  • 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
  • High Days and Holidays in Iceland; Árni Björnsson; Mál og menning, 1995; ISBN 9979-3-0802-8
  • Mapping Time, the calendar and its history; E G Richards; Oxford University Press, 1998; ISBN 0-19-850413-6
  • Sun, Moon, and Sothis; Lynn E. Rose; Kronos Press, 1999; ISBN 0-917994-15-9
  • The Comprehensive Hebrew Calendar; Arthur Spier; Feldheim Publishers, 1986; ISBN 0-87306-398-8
  • Dieter Schuh (1973). Untersuchungen zur Geschichte der Tibetischen Kalenderrechnung. Wiesbaden: Franz Steiner Verlag GMBH.


External links


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