Acalendar is a system of organizingdays. This is done by giving names to periods oftime, typically days,weeks,months andyears.[1][2][3] Adate is the designation of a single and specific day within such a system. A calendar is also a physical record (often paper) of such a system. A calendar can also mean a list of planned events, such as acourt calendar, or a partly or fully chronological list of documents, such as a calendar of wills.
Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycle of thesun or themoon.[4][5][6] The most common type of pre-modern calendar was thelunisolar calendar, a lunar calendar that occasionally adds oneintercalary month to remain synchronized with thesolar year over the long term.
The termcalendar is taken fromkalendae,[7][8] the term for the first day of the month in the Roman calendar, related to the verbcalare 'to call out', referring to the "calling" of the new moon when it was first seen.[9] Latincalendarium meant 'account book, register' (as accounts were settled and debts were collected on the calends of each month).[10] The Latin term was adopted inOld French ascalendier and from there inMiddle English ascalender by the 13th century (the spellingcalendar is early modern).[10]
The course of the Sun and the Moon are the most salient regularly recurring natural events useful fortimekeeping, and in pre-modern societies around the worldlunation and theyear were most commonly used as time units. Nevertheless, the Roman calendar contained remnants of a very ancient pre-Etruscan 10-month solar year.[11]
During theVedic period India developed a sophisticated timekeeping methodology and calendars forVedic rituals.[13] According to Yukio Ohashi, the Vedanga calendar in ancient India was based on astronomical studies during the Vedic Period and was not derived from other cultures.[14]
Calendars in antiquity werelunisolar, depending on the introduction ofintercalary months to align the solar and the lunar years. This was mostly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-centuryColigny calendar.
TheRoman calendar was reformed byJulius Caesar in 46 BC.[19] His "Julian" calendar was no longer dependent on the observation of the new moon, but followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendarmonth fromlunation. TheGregorian calendar, introduced in 1582, corrected most of the remaining difference between the Julian calendar and the solar year.
TheIslamic calendar is based on the prohibition of intercalation (nasi') byMuhammad, in Islamic tradition dated to a sermon given on 9Dhu al-Hijjah AH 10 (Julian date: 6 March 632). This resulted in an observation-based lunar calendar that shifts relative to the seasons of the solar year.
There have been several modern proposals for reform of the modern calendar, such as theWorld Calendar, theInternational Fixed Calendar, theHolocene calendar, and theHanke–Henry Permanent Calendar. Such ideas are promoted from time to time, but have failed to gain traction because of the loss of continuity and the massive upheaval that implementing them would involve, as well as their effect on cycles of religious activity.
A universal calendar, combining different calendars
A full calendar system has a different calendar date for every day.[20][21] Thus the week cycle is by itself not a full calendar system;[22] 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.[23] This applies for theJulian day orUnix Time. 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
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 non-cyclic elements.
Most calendars incorporate more complex cycles. For example, the vast majority of them track years, months, weeks and days. The seven-day week is practically universal, though its use varies. It has run uninterrupted for millennia.[24]
Solar calendars assign adate to eachsolar day.[5] A day may consist of the period betweensunrise andsunset, with a following period ofnight, or it may be a period between successive events such as two sunsets.[25][26] 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 amean solar day. Other types of calendar may also use a solar day.
The Egyptians appear to have been the first to develop a solar calendar,[27] using as a fixed point the annual sunrise reappearance of the Dog Star—Sirius, or Sothis—in the eastern sky, which coincided with the annual flooding of the Nile River.[28][29][30] They built a calendar with 365 days, divided into 12 months of 30 days each, with 5 extra days at the end of the year. However, they did not include the extra bit of time in each year, and this caused their calendar to slowly become inaccurate.[31]
Nearly all calendar systems group consecutive days into "months" and also into "years". In asolar calendar ayear approximates Earth's tropical year (that is, the time it takes for a complete cycle ofseasons), traditionally used to facilitate the planning ofagricultural activities. In alunar calendar, themonth 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 thetropical 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 inleap 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 asintercalation. Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.
Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years. Many cultures use different baselines for their calendars' starting years. Historically, several countries have based their calendars onregnal years, a calendar based on the reign of their current sovereign. For example, the year 2006 inJapan is year 18 Heisei, with Heisei being the era name ofEmperorAkihito.
Anastronomical calendar is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of theSecond Temple. Such a calendar is also referred to as anobservation-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.
Anarithmetic calendar is one that is based on a strict set of rules; an example is the currentJewish calendar. Such a calendar is also referred to as arule-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.
The earlyRoman calendar, created during the reign ofRomulus, lumped the 61 days of the winter period together as simply "winter".[34] Over time, this period became January and February; through further changes over time (including the creation of theJulian calendar) this calendar became the modern Gregorian calendar, introduced in the 1570s.[35][36]
Current usage of civil calendars in the world. Some countries use an additional calendar not listed here that determine holidays and other traditions, for example theChinese orIslamic calendars.
The primary practical use of a calendar is to identify days: to be informed about or to agree on a future event and to record an event that has happened. Days may be significant for agricultural, civil, religious, or social reasons. For example, a calendar provides a way to determine when to start planting or harvesting, which days arereligious orcivil 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 as part of a completetimekeeping system: date and time of day together specify a moment intime. In the modern world, timekeepers can show time, date, and weekday. Some may also show the lunar phase.
TheGregorian calendar is thede facto international standard and is used almost everywhere in the world for civil purposes. The widely used solar aspect is a cycle of leap days in a 400-year cycle designed to keep the duration of the year aligned with thesolar year.[37] There is a lunar aspect which approximates the position of the moon during the year, and is used in thecalculation of the date of Easter.[38] Each Gregorian year has either 365 or 366 days (the leap day being inserted as 29 February), amounting to an average Gregorian year of 365.2425 days (compared to a solar year of 365.2422 days).[39]
The Gregorian calendar was introduced in 1582 as a refinement to theJulian calendar, that had been in use throughout the European Middle Ages, amounting to a 0.002% correction in the length of the year.[37] During the Early Modern period,its adoption was mostly limited toRoman Catholic nations, but by the 19th century it had become widely adopted for the sake of convenience in international trade. The last European country to adopt it was Greece, in 1923.[40]
Thecalendar epoch used by the Gregorian calendar is inherited from the medieval convention established byDionysius Exiguus and associated with the Julian calendar. The year number is variously given as AD (forAnno Domini) or CE (forCommon Era orChristian Era).[41]
A Hindualmanac (pancanga) for the year 1871/2 fromRajasthan (Library of Congress, Asian Division)
The most important use of pre-modern calendars is keeping track of theliturgical year and the observation of religious feast days.
While the Gregorian calendar is itself historically motivated to the calculation of theEaster date, it is now in worldwide secular use as thede facto standard. Alongside the use of the Gregorian calendar for secular matters, there remain several calendars in use for religious purposes.
Western Christian liturgical calendars are based on the cycle of the Roman Rite of the Catholic Church, and generally include the liturgical seasons ofAdvent,Christmas, Ordinary Time (Time afterEpiphany),Lent,Easter, and Ordinary Time (Time afterPentecost). Some Christian calendars do not include Ordinary Time and every day falls into a denominated season.
TheEastern Orthodox Church employs the use of 2 liturgical calendars; theJulian calendar (often called the Old Calendar) and theRevised Julian Calendar (often called the New Calendar). The Revised Julian Calendar is nearly the same as the Gregorian calendar, with the addition that years divisible by 100 are notleap years, except that years with remainders of 200 or 600 when divided by 900 remain leap years, e.g. 2000 and 2400 as in the Gregorian calendar.[42][discuss]
TheIslamic calendar or Hijri calendar is alunar 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. Its epoch is theHijra (corresponding to AD 622). With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately every 33 Islamic years.
Most of the Hindu calendars are inherited from a system first enunciated inVedanga Jyotisha of Lagadha, standardized in theSūrya Siddhānta and subsequently reformed by astronomers such asĀryabhaṭa (AD 499),Varāhamihira (6th century) andBhāskara II (12th century).
TheHebrew calendar is used byJews worldwide for religious and cultural affairs, also influences civil matters in Israel (such asnational holidays) and can be used business dealings (such as for the dating ofcheques).[43]
Followers of theBaháʼí Faith use theBaháʼí calendar. The Baháʼí Calendar, also known as the Badi Calendar was first established by the Bab in the Kitab-i-Asma. The Baháʼí Calendar is also purely a solar calendar and comprises 19 months each having nineteen days.
TheIranian (Persian) calendar is used inIran and some parts ofAfghanistan. TheAssyrian calendar is in use by the members of the Assyrian community in the Middle East (mainly Iraq, Syria, Turkey, and Iran) and the diaspora. The first year of the calendar is exactly 4750 years prior to the start of the Gregorian calendar. TheEthiopian calendar or Ethiopic calendar is the principal calendar used inEthiopia andEritrea, with theOromo calendar also in use in some areas. In neighboringSomalia, theSomali calendar co-exists alongside the Gregorian and Islamic calendars. InThailand, where theThai solar calendar is used, the months and days have adopted the western standard, although the years are still based on the traditionalBuddhist calendar.
A fiscal calendar generally means the accounting year of a government or a business. It is used for budgeting, keeping accounts, and taxation. It is a set of 12 months that may start at any date in a year. The US government's fiscal year starts on 1 October and ends on 30 September. The government of India's fiscal year starts on 1 April and ends on 31 March. Small traditional businesses in India start the fiscal year onDiwali festival and end the day before the next year's Diwali festival.
In accounting (and particularly accounting software), a fiscal calendar (such as a4/4/5 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 (theISO 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.
The termcalendar applies not only to a given scheme of timekeeping but also to a specific record or device displaying such a scheme, for example, anappointment book in the form of a pocket calendar (orpersonal organizer), desktop calendar, awall calendar, etc.
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.
In the US Sunday is considered the first day of the week and so appears on the far left and Saturday the last day of the week appearing on the far right. In Britain, the weekend may appear at the end of the week so the first day is Monday and the last day is Sunday.[citation needed] The US calendar display is also used in Britain.
It is common to display the Gregorian calendar in separate monthly grids of seven columns (from Monday to Sunday, or Sunday to Saturday depending on which day is considered to start the week – this varies according to country)[citation needed] and five to six rows (or rarely, four rows when the month of February contains 28 days incommon years beginning on the first day of the week), with the day of the month numbered in each cell, beginning with 1. The sixth row is sometimes eliminated by marking 23/30 and 24/31 together as necessary.
When working with weeks rather than months, a continuous format is sometimes more convenient, where no blank cells are inserted to ensure that the first day of a new month begins on a fresh row.
Birashk, Ahmad (1993),A Comparative Calendar of the Iranian, Muslim Lunar, and Christian Eras for Three Thousand Years, Mazda Publishers,ISBN978-0-939214-95-2
Blackburn, Bonnie; Holford-Strevens, Leofranc (2003) [1999],The Oxford Companion to the Year (corrected reprinting of 1st ed.), Oxford University Press
Doggett, L.E. (1992),"Calendars", in Seidelmann, P. Kenneth (ed.),Explanatory Supplement to the Astronomical Almanac, University Science Books,ISBN978-0-935702-68-2
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