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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 BC 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 BC (67 days between November and December, 23 at the end of February). This caused the spring of 45 BC 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 BC 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, descr='[AH]' 1, was July 16, 622, in the Western calendar (descr='[AH]' [Anno Hegirae=in the year of the Hegira] is used to indicate the Islamic year). Muharram 1, descr='[AH]' 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. BC 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).

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A calendar is a system of reckoning and ordering time beyond the period of a day in a repetitive, usually annual, cycle. A calendar's primary function is regulating and organizing human activities; the word derives from the Latin calendarium or calendra, "account book," and kalendae or "calends," the new moon and first day of the Roman month, when Romans paid their debts. Calendars may have derived from the human penchant for imposing order; however, the most efficient exploitation of natural resources implies synchronizing productive efforts with nature's cycles. Sensitivity to such cycles is biologically programmed into humans as circadian rhythms, including the twenty-four-hour cycle of sleep and wakefulness and fluctuating body temperature; and in the female menstrual cycle, which approximates a lunar period.

Calendric periodicities are traced ultimately to the Sun and the Moon. The daily apparent rising and setting of the Sun is due to the rotation of the Earth, while the annual cycle of the seasons is related to the revolution of the Earth around the Sun, and the tilt of the Earth relevant to its plane of revolution. The most commonly reckoned calendrical period beyond day and night is the synodical lunar month (the cycle of lunar phases) of 29.5 days. Incommensurability between this period and the seasonal cycle based on the solar year of 365.24 days and the need to process fractions of days in the astronomical cycles with whole-day counts have been among the most difficult challenges for calendar specialists.

Early, Nonliterate, and Folk Calendars

Alexander Marshack sees in the scorings and tally marks on Paleolithic fossils and artifacts the beginnings of time recording. With Neolithic domestication, systematic time reckoning allowed farming practices to fit local moisture and temperature patterns. Such concerns and efforts are suggested at Stonehenge in England, where, beginning about five thousand years ago, massive stones were arranged in geometric patterns. While interpretations of the site vary, its main axis includes an alignment to the June solstice sunrise, the day of longest sunlight.

Small-scale and nonliterate societies such as the Nuer of Africa emphasize a sensitivity and responsiveness to seasonal cycles that E. E. Evans-Pritchard calls "ecological time." Such calendars are characterized by:

  1. space-time (the fusing of concepts of time with the space accessed and occupied in that time);
  2. fuzzy-bordered seasons (increasing rains gradually transform dry season to wet);
  3. "layered" observations of synchronous events at different ecological levels (for example, the bloom of plant species coincident with the movement of fish or game); and
  4. diachronic sequences, such as episodic faunal or floral changes or consecutive astronomical observations (for example, advancing stellar positions, changes in the Moon's position and shape, or the movement of the Sun on the horizon).

Calendar Codification and Civilization

Awareness of the astronomical significance behind seasonal phenomena allowed human communities to coordinate their activities seasonally for strategic and productive ends. Once a reliable system of recording such information was devised, calendar refinement and codification were possible. Calendar codification and the enhanced utilization of energy and other resources that this enabled are a significant factor in the process of civilizationa topic deserving additional study. The earliest centers of civilization in Mesopotamia, the Indus and Nile valleys, eastern China, Mesoamerica, and the Andes all have information recording systems and codified calendars, which were probably overseen by high-ranking astronomer-priests who also likely oversaw timely rituals relating human endeavors to cosmic powers.

Varieties of Calendars

The prominence of the Moon in premodern societies with limited lighting and the regularity of its phases resulted in the synodical lunar month being basic to many traditional calendars. In the Muslim calendar, twelve lunar months of twenty-nine or thirty days are reckoned in a year of 354 days, or a leap year of 355, in a thirty-year cycle. The approximately eleven-day difference between such a synodical lunar calendar and the solar year, however, results in a slippage of months through the seasons. In order to maintain synchrony between lunar months and the seasons, an intercalated month is necessary, a strategy employed in the Jewish calendar with influence from Babylonia: seven leap years intersperse with twelve common years in a nineteen-year cycle.

Ancient Egyptians used the annual heliacal rise (predawn reappearance) of Sirius to help coordinate their lunar months with the seasons and solar year. While maintaining this system for religious observance, they later developed a civil year of 365 days comprising twelve fixed months of thirty days with five additional days. The ancient Maya had a similar five-day end-of-year, but divided the other 360 days into eighteen named periods of twenty days. This yearly calendar intermeshed every fifty-two years with a divinatory cycle of 260 days. Maintaining separate calendars for civic and religious (and/or regional and ethnic) functions is a common practice, useful in the twenty-first century for the retention of local traditions amidst the spread of the Gregorian calendar.

The Gregorian Calendar and Globalization

The Gregorian calendar spread through European colonialism and later through international relations, exchange, and commerce. It developed from the first-century b.c.e. Julian calendar, named after Julius Caesar, who commissioned its development and approved the reckoning of months no longer determined by lunar observations in a year that averaged 365.25 days. The Gregorian gets its name from Pope Gregory XIII, by whose election in the year 1572 c.e. the day marking the vernal equinox had strayed ten days from its occurrence. His papal bull in 1582 set out the mechanisms by which:

  1. the spring equinox would occur on its actual date;
  2. Easterwhich depends on calculations from the vernal equinoxwould be determined; and
  3. a leap year system would be implemented, allowing for prolonged congruency between the calendar and its astronomical underpinnings.

Since the sixteenth century, countries around the world have adopted the Gregorian calendar, including its twelve fixed months, seven-day weeks, and beginning date. However, day and month names usually occur in the vernacular, and traditional reckonings may be kept for local, ethnic, and religious observances.

Through calendars, humans impose culturally significant rhythms on the perception of time. Across human cultures two primary perceptions of the character of time predominate:

  1. time as recursive or cyclic, observed in the recurrence of day and night, lunar waxing and waning, and the return of the seasons; and
  2. time as linear, an ongoing process, observed in the maturation of vegetation, decay, and the transition of a human life from birth to death.

Specially marked dates and periodicities are the human, cultural cadence in the infinitude of time. While unable to control the passage of time, humans with calendars have increasingly ordered their relationship to and utilization of it.

See also Astronomy, Pre-Columbian and Latin American ; Time .


Aveni, Anthony. Empires of Time: Calendars, Clocks, and Cultures. Boulder: University Press of Colorado, 2002.

Evans-Pritchard, E. E. The Nuer: A Description of the Modes of Livelihood and Political Institutions of a Nilotic People. Oxford: Oxford University Press, 1940. Reprint, 1969.

Fabian, Stephen Michael. Space-Time of the Bororo of Brazil. Gainesville: University Press of Florida, 1992.

Marshack, Alexander. The Roots of Civilization: The Cognitive Beginnings of Man's First Art, Symbol and Notation. London: Weidenfeld and Nicolson, 1972.

Richards, Edward G. Mapping Time: The Calendar and Its History. Oxford: Oxford University Press, 1998.

Westrheim, Margo. Calendars of the World: A Look at Calendars and the Ways We Celebrate. Oxford: Oneworld Publications, 1994.

Stephen M. Fabian

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The Jewish calendar is fixed according to the number of years since the creation of the world (traditionally 3761 BCE. Thus the year 5000 began on 1 Sept. 1239 CE. When using the secular calendar, Jews use the terms BCE (before common era) and CE (common era) rather than BC and AD. The year follows a 354 day year of twelve lunar months. To harmonize this with the solar year of 365 1/4 days, an extra month, Adar II, is added into seven of every nineteen years. The months received Babylonian names during the Exile: Tishri (Sept./Oct.), Heshvan (Oct./Nov.), Kislev (Nov./Dec.), Tevet (Dec./Jan.), Shevat (Jan./Feb.), Adar (Feb./Mar.), Adar II (see above), Nisan (Mar./Apr.), Iyyar (Apr./May), Sivan (May/June), Tammuz (June/July), Av (July/Aug.), Elul (Aug./Sept.). The year begins with 1 Tishri, Rosh ha-Shanah.

A day begins and ends at sunset. Rosh ha-Shanah (the new year) is kept on 1 Tishri. It is followed by the days of repentance and Yom Kippur on 10 Tishri. The season of Sukkot (tabernacles) begins on 15 Tishri and concludes with Shemini Azeret (the Closing Festival) and Simḥat Torah (the rejoicing in the law) on 22/23 Tishri. Ḥanukkah (Lights) begins on 25 Kislev and ends on 2 Tevet. 10 Tevet is a fast day and 15 Shevat is the new year for trees. Purim (Lots, the Feast of Esther) is celebrated on 14 Adar. It is preceded by the Fast of Esther (13 Adar) and succeeded by Shushan Purim (15 Adar). Pesaḥ (Passover) begins on 15 Nisan and ends on 21/22 Nisan. 27 Nisan is Yom ha-Shoʾah (Day of the Holocaust) and 5 Iyyar is Israel Independence Day. Lag ba-Omer (the thirty-third day of the counting of the omer) is celebrated on 18 Iyyar and Shavuot (Pentecost) takes place on 6/7 Sivan. There are fast days on 17 Tammuz and 9 Av, and 15 Av is a minor holiday.


The Julian calendar was reformed by Pope Gregory XIII in 1582 when it was realized that the Christian calendar was ten days in advance of the solar year. The reformed calendar is known as the Gregorian (or New) Style, the unreformed as the Julian (or Old) Style. The difference between the two calendars is now thirteen days, so that some Orthodox observe Christmas, 25 Dec. (Old Style), on 7 Jan. The Christian calendar follows each year the preparation for the coming of Christ, his life, death, and resurrection, and the being of God (see FESTIVALS AND FASTS). Thus, it begins with Advent, which has four Sundays, and then either one or two Sundays after Christmas bridge the gap to the Epiphany (6 Jan.). Thereafter ‘Sundays after Epiphany’ are reckoned until what used to be known as Septuagisma, Sexagesima, and Quinquagesima (Sundays before Lent); Ash Wednesday introduces the forty days of Lent, with its six Sundays; and five Sundays after Easter lead up to Ascension day with its following Sunday and Pentecost (Whitsunday). The remaining Sundays until Advent are numbered ‘after Trinity’ or ‘after Pentecost’. The Sundays of the Orthodox year fall into three segments: triodion (the ten weeks before Easter), pentecostarion (the paschal season), and octoechos (the rest of the year). See also FESTIVALS AND FASTS.

The system of dating years AD (Lat., Anno Domini, ‘in the year of the Lord’) goes back to Dionysius Exiguus (‘the Small’; c.500–50). The abbreviations CE (Common Era) and BCE to replace AD and BC began with Jewish historians in the 19th cent., in order to avoid a religious confession within the words abbreviated.


The Muslim calendar is lunar, with twelve months of twenty-nine or thirty days. Because this is not adjusted to the solar calendar (contrast the Jewish system), the religious festivals and holidays advance around the seasons: thus the month of fasting, Ramaḍān, moves around the entire solar year, occurring sometimes in summer and sometimes in winter (intercalation is forbidden in the Qurān 9. 37). The months are: Muḥarram; Ṣafr, Rabīʿ al-Awwal, Rabī al-Thāni, Jumādā al-Ūlā, Jumādā al-Thāniyya, Rajab, Shaʿbān, Ramadhān, Shawwal, Dhū al-Qadah, Dhū al-Ḥijjah. The years are numbered from the Hijra, the move of the Prophet Muḥammad from Mecca to Madīna in 622 CE. 1 Muḥarram of that year was 16 July 622, which begins the first year of the Muslim era. The years are referred to as AH, i.e. ‘after the Hijra’.


The Hindu religious calendar is lunar, with the months divided into a bright (śulapakṣa) and a dark (kṛṣṇapakṣa) half, with fifteen tithis (days) in each. The correlation of human activity with the whole cosmic process (made evident in the movement of heavenly bodies) is of paramount importance. The religious calendar is then a proliferation of special observances, for some of which see FESTIVALS AND FASTS. There are six seasons (ṛtu): (i) Vasanta (spring); (ii) Grīṣma (hot season); (iii) Varṣa (rainy season); (iv) Śarad (autumn); (v) Hemanta (winter); (vi) Śiśira (cold). To each of these is allocated two months (Caitra, Vaiśākha; Jyaiṣtha, Aṣāḍha; Śrāvana, Bhādarapada; Aśvinā, Āśvayuja; Mārgaśīrṣa, Pauṣa; Māgha, Phālguna. Every two or three years a thirteenth month was added to adjust the lunar year to the solar year.


The spread of Buddhism did not take with it a calendar which it then imposed on other countries; rather, it adapted to local calendars, and worked its own festivals into the local scene. Buddhist calendars thus vary from culture to culture.


The Sikhs' religious calendar is a modified form of the Bikramī calendar. The year is solar (23 minutes 44 seconds shorter than the Christian year) and the months are lunar. Lunar month dates, varying within fifteen days, are used for gurpurbs. So in 1984 Gurū Gobind Siṅgh's birthday fell on both 10 Jan. and 29 Dec. Solar months, based on the twelve zodiac signs, are also used, e.g. for saṅgrānds, Baisākhī, and Lohṛī. The anniversaries of the battle of Chamkaur, martyrdom of the younger sāhibzāde, and battle of Muktsar are solar dates. Because of the discrepancy between the Bikramī and Christian solar year these dates advance one day in sixty-seven years.


The Chinese have traditionally followed both a solar and a lunar calendar. These run concurrently and coincide every nineteen years. The solar calendar divides the year into twenty-four periods, named (mainly) according to the weather expected in that period in the N. China plain. The only festival fixed by the solar calendar is at the beginning of the fifth period, Chʾing Ming. The lunar calendar is used to record public and private events. The New Year begins with the second new moon after the winter solstice, between 21 Jan. and 20 Feb. The months have no names and are known by numbers; but they are associated with the five elements of the cosmos, wood, fire, earth, metal, and water; and also with animals; hence each year is known as ‘the year of’. Thus 2000 is the year of the dragon; 2001 the snake; 2002 the horse; 2003 the sheep; 2004 the monkey; 2005 the chicken; 2006 the dog; 2007 the pig; 2008 the rat; 2009 the ox; 2010 the tiger. The traditional starting-point for chronological reckoning is the year in which the minister of the emperor, Huang-ti, worked out the sixty-year cycle, i.e. 2637 BCE.



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CALENDAR. It was widely recognized in the early sixteenth century that the calendar was inaccurate, but the question of how it should be reformed and who had the authority to do so raised fundamental issues. It was some two hundred and fifty years before all of Europe had changed.

The Christian Church had adopted the Julian calendar from the Roman Empire at the Council of Nicaea in 325 c.e.: the first general council of the church, its authority acknowledged thereafter by East and West, Protestants and Catholics. A slight error in the original Roman calculations had by 1500 accumulated to ten days, leaving the real spring equinox on 11 March instead of 21 March. What really bothered the Roman Catholic Church (though not, apparently, the Orthodox Church) was the error this produced in the date of Easter. This was supposed to fall on the Sunday on or after the full moon after 21 March, but it now often fell a month late relative to the real equinox. Nicolaus Copernicus's De Revolutionibus Orbium Caelestium (1543; On the revolutions of the celestial orbs) had originally been commissioned as a basis upon which to reform the calendar, but the intervening Reformation and Copernicus's heretical views about the solar system overlaid the issue.

One of the last acts of the Counter-Reformation Council of Trent was to order a reform of the calendar, which it was hoped would provide a basic measure of agreement between Protestants and Catholics on at least one fundamental issue. The observations and calculations were undertaken by the Jesuit astronomer Christoph Clavius (15371612), and the results embodied in Pope Gregory XIII's bull of 1582. Ten days were to be removed from October 1582 to bring the calendar back in line with the seasons, and the system of leap years was modified to keep it on track; from then on there was to be a leap year only at the end of every fourth century, and not of every century as before. The old formula for calculating the date of Easter was modified but retained. The Gregorian reform was fundamentally religious rather than astronomical, and the Roman Catholic Church continued to reject Copernicus.

Only a handful of countries (Spain, Portugal, Poland, and parts of Italy) adopted the new Gregorian calendar on time, not least because the bull was promulgated so late. By 1585 most Roman Catholic countries had followed. Most Protestant statesincluding large parts of Switzerland, Germany, the Protestant Low Countries, Great Britain, and Scandinaviaretained the Julian calendar for another century or more, creating a patchwork of calendrical practice throughout Europe, particularly complex in the Holy Roman Empire. The key issue was not astronomical accuracy but papal authority. By accepting a papal bull, states would appear to be recognizing the authority of the pope not only to interfere in civil affairs but also to alter decisions of the early church; indeed, most Roman Catholic countries took care to adopt the new calendar by their own civil acts. In England, the mathematician and astrologer John Dee (15271608) argued that the time of Christ, rather than that of the early church, was the appropriate "radix of time" for Protestants, and proposed his own Elizabethan imperial calendar one day ahead of Rome, but his views were unwelcome to the authorities and in the end England did nothing.

In 1700, with the gap between the two calendars set to widen to eleven days, most Protestant states followed a resolution of the imperial Diet of Regensburg and adopted a modified version of the Gregorian calendar. They did so using their own calculations, following the German astronomer Johannes Kepler (15711630), and substituting an astronomical Easter for the traditional version, to the same practical effect. In Britain, where antipopery remained strong, the new calendar was not adopted until September 1752, when eleven days were omitted and a third Easter calculation adopted, also to identical effect. Sweden pursued its own course, coming fully into line in 1753. The churches of the East remained unmoved, standing fast by the decisions of early Christendom; the fast-secularizing states of eastern Europe generally went Gregorian for civil purposes around the time of World War I.


Did the calendar change create practical, as opposed to political, problems? Undoubtedly it did, especially in international communications and where Protestant and Catholic jurisdictions were interspersed, as in much of central Europe and the Low Countries. The modest disruption of the familiar relationship between the feasts of the church and the seasons was quite quickly overcome, but the actual details varied according to how the reform was implemented. In Britain in 1752, for example, the eleven days September 313 inclusive were omitted from the calendar, bringing human events eleven days forward in the natural year. Fairs however were left at the same place in the natural year, putting their calendar dates back by eleven days (although many fairs in practice moved forward). Financial payments too kept their full natural term, leaving the financial year ending on 5 April rather than the traditional 25 March. At the same time, the start of the legal year was altered from 25 March to 1 January. The arrival of the new Christmas Day eleven days early took many by surprise in a society that still reckoned by feasts and fairs as much as by dates and diaries. There was widespread resistance and resentment, although the tale that people rioted for their eleven lost days is a myth. In Bohemia and in Augsburg, though, there were several years of strife between Catholics and Protestants over the issue in the 1580s, known as the "Kalenderstreit."

In navigating between old-style and new-style calendars, it is necessary to remember that in general Roman Catholic states were ten days ahead of Protestant and Orthodox states from 1583 until 1700. Care must be taken in the 1580s, and with Britain, Sweden, the Netherlands, and Switzerland. Catholic minorities in Protestant states may have adopted either calendar for religious purposes. For clarity, historians often note "O.S." or "N.S." after Julian and Gregorian dates respectively.

The issue of the calendar is a reminder that the reference points for the calculation of time express the most basic assumptions of society. The disputes it engendered were symptomatic of religious and political divisions in a world where nothing could be taken for granted.

See also Copernicus, Nicolaus ; Dee, John ; Kepler, Johannes ; Time, Measurement of ; Trent, Council of .


Cheney, C. R. A Handbook of Dates for Students of British History. Rev. ed. London, 2000.

Coyne, G. V., M. A. Hoskin, and O. Pedersen, eds. Gregorian Reform of the Calendar: Proceedings of the Vatican Conference to Commemorate its Four Hundredth Anniversary, 15821982. Vatican City, 1983.

Poole, Robert. Time's Alteration: Calendar Reform in Early Modern England. London, 1998.

Richards, E. G. Mapping Time: The Calendar and Its History. Oxford and New York, 1998.

Whitrow, G. J. Time in History: The Evolution of Our General Awareness of Time and Temporal Perspective. Oxford, 1988.

Robert Poole

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A calendar is a system of measuring the passage of time for the purpose of recording historic events and arranging future plans. Units of time are defined by three different types of motion: a day is one rotation of Earth around its axis; a month is one revolution of the Moon around Earth; and a year is one revolution of Earth around the Sun. The year is the most important time unit in most calendars, since the cycle of seasons repeat in a yearly cycle as Earth revolves around the Sun.

Making a yearly calendar, however, is no simple task because these periods of time do not divide evenly into one another. For instance, the Moon completes its orbit around Earth (a lunar month) in 29.5 days. A lunar year (12 lunar months) equals 365 days, 8 hours, and 48 minutes. A solar year (time it takes Earth to complete its orbit around the Sun) is 365.242199 days, or 365 days, 5 hours, 48 minutes, and 46 seconds. The calendar we currently use is adjusted to account for the extra fraction of a day in each year.

Development of the present-day calendar

The official calendar currently used worldwide is the Gregorian calendar. The ancient Egyptians adopted a 365-day calendar sometime between 4000 and 3000 b.c. The first major improvement to that 365-day calendar was made by Roman dictator Julius Caesar (10044 b.c.) in 46 b.c. With the help of Greek astronomer Sosigenes, Caesar developed a calendar divided into 12 months of 30 and 31 days, with the exception of 29 days in February. In this new Julian calendar (named after Caesar), an extra day, or leap day, was added to every fourth year to account for the 365.25-day solar year.

The Julian calendar, however, was still off by 11 minutes and 14 seconds each year. Over 300 years, this difference added up to just over 3 days. By the mid-1500s, the Julian calendar was another 10 days ahead of Earth's natural yearly cycle.

To adjust this calendar to line up with the seasons, Pope Gregory XIII (15021585) introduced another change in 1582. He first ordered that 10 days be cut from the current year, so that October 4, 1582, was followed by October 15, 1582. He then devised a system whereby three days are dropped every four centuries. Under the original Julian calendar, every century year (200, 300, 400, etc.) was a leap year. In the new calendar, named the Gregorian calendar, only those century years divisible by 400 (800, 1200, 1600, etc.) are leap years.

Although not perfect, the Gregorian calendar is accurate to within 0.000301 days (26 seconds) per year. At this rate, it will be off one day by about the year 5000.

The Jewish and Muslim calendars

The Jewish calendar is a lunisolar calendar, a combination of lunar and solar years. The calendar is 12 lunar months long, with an additional month added every few years to keep the calendar in line with the seasons. The months (with corresponding days) are Tishri (30), 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). Adar II, the extra month, is added periodically after Adar. Calendar years vary from 353 to 355 days; leap years may have 383 to 385 days. Rosh ha-Shanah, the Jewish New Year, is observed on the first day of Tishri.

The Muslim calendar is a strict lunar calendar. Calendar years vary from 354 to 355 days, with the months and seasons having no connection. The months are Muharram (30), Safar (29), First Rabia (30), Second Rabia (29), First Jumada (30), Second Jumada (29), Rajab (30), Shaban (29), Ramadan (30), Shawwal (29), Dhu-I-Kada (30), and Dhu-I-Hijja

(29 or 30). The first day of the first year of the Muslim calendar corresponds to July 16, 622, of the Gregorian calendar.

The Gregorian calendar and the third millennium

Technically, the year 2000 on the Gregorian calendar was not the beginning of the third millennium (a 1,000-year span). In actuality, it was the last year of the second millennium. When the Gregorian calendar was adopted, the transition of the years b.c. to a.d.marking the birth of Jesus of Nazarethdid not include the year 0. The sequence runs 2 b.c., 1 b.c., a.d. 1, a.d. 2, etc. According to this sequence, since there is no year zero, the first year of the first millennium was a.d. 1. Thus, the first day of the third millennium was January 1, 2001.

Possible future calendar reform

Although the Gregorian calendar allows for the oddity of Earth's orbit and for the dates when Earth is closest and farthest from the Sun, the shortness of February introduces slight problems into daily life. For example, a person usually pays the same amount of rent for the 28 days of February as is paid for the 31 days of March. Also, the same date falls on different days of the week in different years. These and other examples have led to several suggestions for calendar reform.

Perhaps the best suggestion for a new calendar is the World Calendar, recommended by the Association for World Calendar Reform. This calendar is divided into four equal quarters that are 91 days (13 weeks) long. Each quarter begins on a Sunday on January 1, April 1, July 1, and October 1. These 4 months are each 31 days long; the remaining 8 months all have 30 days. The last day of the year, a World Holiday (W-Day), comes after December 30 (Saturday) and before January 1 (Sunday) of the new year. W-Day is the 365th day of ordinary years and the 366th day of leap years. The extra day in leap years would appear as a second World Holiday (Leap year or L-Day) between June 30 (Saturday) and July 1 (Sunday). The Gregorian calendar rules for ordinary, leap, century, and non-century years would remain unchanged for the foreseeable future.

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In Russia, the calendar has been used not only to mark the passage of time, but also to reinforce ideological and theological positions. Until January 31, 1918, Russia used the Julian calendar, while Europe used the Gregorian calendar. As a result, Russian dates lagged behind those associated with contemporary events. In the nineteenth century, Russia was twelve days behind, or later than, the West; in the twentieth century it was thirteen days behind. Because of the difference in calendars, the Revolution of October 25, 1917, was commemorated on November 7. To minimize confusion, Russian writers would indicate their dating system by adding the abbreviation "O.S." (Old Style) or "N.S." (New Style) to their letters, documents, and diary entries.

The Julian Calendar has its origins with Julius Caesar and came into use in 45 b.c.e. The Julian Calendar, however, rounded the number of days in a year (365 days, 6 hours), an arithmetic convenience that eventually accumulated a significant discrepancy with astronomical readings (365 days, 5 hours, 48 minutes, 46 seconds). To remedy this difference, Pope Gregory XIII introduced a more accurate system, the Gregorian Calendar, in 1582.

During these years Russia had used the Byzantine calendar, which numbered the years from the creation of the world, not the birth of Christ, and began each new year on September 1. (According to this system, the year 7208 began on September 1, 1699.) As part of his Westernization plan, Peter the Great studied alternative systems. Although the Gregorian Calendar was becoming predominant in Catholic Europe at the time, Peter chose to retain the Julian system of counting days and months, not wanting Orthodox Russia to be tainted by the "Catholic" Gregorian system. But he introduced the numbering of years from the birth of Christ. Russia's new calendar started on January 1, 1700, not September 1. Opponents protested that Peter had changed "God's Time" by beginning another new century, for Russians had celebrated the year 7000 eight years earlier.

Russians also used calendars to select names for their children. The Russian Orthodox Church assigned each saint its own specific feast day, and calendars were routinely printed with that information, along with other appropriate names. During the imperial era, parents would often choose their child's name based on the saints designated for the birth date.

Russia continued to use the Julian calendar until 1918, when the Bolshevik government made the switch to the Gregorian system. The Russian Orthodox Church, however, continued to use the Julian system, making Russian Christmas fall on January 7. The Bolsheviks eliminated some confusion by making New Year's Day, January 1, a major secular holiday, complete with Christmas-like traditions such as decorated evergreen trees and a kindly Grandfather Frost who gives presents to children. Christmas was again celebrated in the post-communist era, in both December and January, but New Year's remained a popular holiday.

See also: old style


Hughes, Lindsey. (1998). Russia in the Age of Peter the Great. New Haven, CT: Yale University Press.

Ann E. Robertson

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65. Calendar

See also 11. ALMANACS ; 396. TIME

a flgure-of-eight-shaped scale, for showing the declination of the sun and the equation of time for every day of the year. analemmatic, adj.
the twenty-ninth day of February, added to the calendar every four years, except in centenary years evenly divisible by 400, to compensate for the discrepancy between the arbitrary 365-day calendar year and the actual time of the solar year. bissextile, adj.
Rare. a person who makes calendars.
1. an intercalation of a day or days in the calendar to correct error.
2. the day or days intercalated. embolic, embolismic, embolismical, adj.
the study of the origin, growth, meaning, and history of Christian religious feasts. heortological, adj.
in the Roman Empire, the cyclical, fifteen-year fiscal period, used for dating ordinary events. Also called cycle of indiction. indictional. adj.
inserted into the calendar, as the twenty-ninth day of February in a leap year. intercalation, n. intercalative, adj.
the period of the moons synodic revolution, from the time of the new moon to the next new moon; one lunar month or approximately 29 1/2 days.
lustrum, luster, lustre
a period of five years.
1. a list or calendar of months.
2. Eastern Orthodoxy. a calendar of all festivals for martyrs and saints, with brief accounts of their lives. Also Menologion.
2. a church calendar, listing festivals for saints.
the practice of eliminating the bissextile day every 134 years to adjust the date of the new moon. Cf. proemptosis.
1. the time of the new moon or the beginning of the month.
2. a heathen festival at the time of the new moon.
the adding of a day every 300 and again every 2400 years to adjust the date of the new moon. Cf. metemptosis.

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calendar Way of reckoning time for regulating religious, commercial and civil life, and for dating events in the past and future. Ancient Egyptians had a system based on the movement of the star Sirius and on the seasons. Calendars are based on natural and astronomical regularities: tides and seasons, movements of the Sun and Earth, and phases of the Moon. The basic units are day, month and year. The main difficulty in compiling a calendar is that the month is not an exact number of days and the year not an exact number of months. For convenience, months and days are assigned a whole number of days, and extra days (intercalations) are added at intervals to compensate. In the modern Gregorian, or New Style, calendar, an extra day (February 29) is added every four years (leap year). The Gregorian calendar was based on the Julian, or Old Style, solar calendar. This was introduced by Julius Caesar in the 1st century bc and was developed from an earlier Moon-based calendar. The Jewish calendar is semilunar, made up of 12 common years and 7 leap years. The leap years have an additional month (Adar II). The Muslim calendar is wholly lunar, the year always consisting of 12 months without intercalations. The months have alternately 30 and 29 days, except for the twelfth month (Dulhajj) which has one intercalary day in 11 years out of a cycle of 30 calendar years.

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Chinese calendar

Chinese calendar


Years (19012008)

Although officially banned in 1930 the ancient Chinese calendar is still widely used in China, and the New Year (the second new moon after the beginning of winter) is a national holiday. It also remains in use in Tibet, Malaysia and other parts of Southeast Asia. Based on the lunar year, it comprises 12 months of 29 or 30 days, each starting with a new moon. A month is repeated seven times during each 19-year cycle.

Buffalo or Cow
















































































Rooster or Chicken








































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cal·en·dar / ˈkaləndər/ (abbr.: cal or cal.) • n. a chart or series of pages showing the days, weeks, and months of a particular year, or giving particular seasonal information. ∎  a datebook. ∎  a system by which the beginning, length, and subdivisions of the year are fixed. ∎  a timetable of special days or events of a specified kind or involving a specified group: the college calendar. ∎  a list of people or events connected with particular dates, esp. canonized saints and cases for trial. • v. [tr.] enter (something) in a calendar or timetable. DERIVATIVES: ca·len·dric / kəˈlendrik/ adj. ca·len·dri·cal / kəˈlendrikəl/ adj.

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A list of cases that are awaiting trial or other settlement, often called a trial list or docket.

A special calendar is an all-inclusive listing of cases awaiting trial; it contains dates for trial, names of counsel, and the estimated time required for trial. It is maintained by a trial judge in some states and by a court clerk in others.

Calendar call is a court session during which the cases that await trial are called in order to determine the current status of each case and to assign a trial date.

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an orderly list of persons, things, or events; a list of offenders in the Newgate Calendar or in similar prisons or at Quarter Session Courts; a list or record.

Examples: calendar of academics; of crimes, 1856; of documents; of my past endeavours, 1601; of martyrs, 1781; of saints, 1631; of sins, 1633.

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calendar system of divisions of the civil year XIII; table showing these XIV. ME. kalender — AN. calender, OF. calendier (mod. calendrier) — L. kalendārium account-book, f. kalendæ CALENDS, the day on which accounts were due.

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calendarbidder, consider, Jiddah, kidder, whydah •bewilder, builder, guilder, Hilda, Matilda, St Kilda, Tilda, tilde •Belinda, Cabinda, cinder, Clarinda, Dorinda, hinder, Kinder, Linda, Lucinda, Melinda, tinder •Drogheda • shipbuilder • bodybuilder •coachbuilder • boatbuilder • Candida •spina bifida •calendar, calender •Phillida • cylinder • Phasmida •Andromeda • Mérida • Florida •Cressida • lavender • provender •chider, cider, divider, eider, glider, Guider, Haida, hider, Ida, insider, Oneida, outsider, provider, rider, Ryder, Saida, slider, spider, strider, stridor •Wilder •binder, blinder, finder, grinder, kinda, minder, ringbinder, winder •Fassbinder • spellbinder • highbinder •bookbinder • pathfinder •rangefinder • viewfinder • backslider •paraglider • childminder • outrider •joyrider • roughrider • ringsider •Tynesider • sidewinder

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