A CALENDAR is a system of organising days for social, religious, commercial or administrative purposes. This is done by giving names to periods of time , typically days , weeks , months and years . A date is the designation of a single, 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 a court 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, synchronised with the cycle of the sun or the moon . The most common type of pre-modern calendar was the lunisolar calendar , a lunar calendar that occasionally adds one intercalary month to remain synchronised with the solar year over the long term.
The three principal calendars are the Gregorian , Jewish , and Islamic calendars. The term _calendar_ itself is taken from _calendae _, the term for the first day of the month in the Roman calendar, related to the verb _calare_ "to call out", referring to the "calling" of the new moon when it was first seen. Latin _calendarium_ meant "account book, register" (as accounts were settled and debts were collected on the calends of each month). The Latin term was adopted in Old French as _calendier_ and from there in Middle English as _calender_ by the 13th century (the spelling _calendar_ is early modern).
* 1 History
* 1.1 Modern reforms
* 2 Calendar systems
* 2.1 Solar calendars * 2.2 Lunar calendars * 2.3 Lunisolar calendars
* 3 Calendar subdivisions
* 4 Other calendar types
* 4.1 Arithmetic and astronomical calendars * 4.2 Complete and incomplete calendars
* 5 Calendars in use
* 5.1 Gregorian calendar * 5.2 Religious calendars * 5.3 National calendars * 5.4 Fiscal calendars
* 6 Formats
* 6.1 Calendaring software
* 7 See also
* 8 References
* 8.1 Citations * 8.2 Sources
* 9 Further reading * 10 External links
The course of the Sun and the Moon are the most evident forms of timekeeping , and the year and lunation were most commonly used in pre-modern societies worldwide as time units. Nevertheless, the Roman calendar contained very ancient remnants of a pre-Etruscan 10-month solar year. The first recorded calendars date to the Bronze Age , dependent on the development of writing in the Ancient Near East , the Egyptian and Sumerian calendars.
A larger number of calendar systems of the Ancient Near East becomes accessible in the Iron Age , based on the Babylonian calendar . This includes the calendar of the Persian Empire, which in turn gave rise to the Zoroastrian calendar as well as the Hebrew calendar .
A great number of Hellenic calendars develop in Classical Greece , and with the Hellenistic period also influence calendars outside of the immediate sphere of Greek influence, giving rise to the various Hindu calendars as well as to the ancient Roman calendar .
Calendars in antiquity were lunisolar , depending on the introduction of intercalary 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-century Coligny calendar .
The Roman calendar was reformed by Julius Caesar in 45 BC. The Julian calendar was no longer dependent on the observation of the new moon but simply followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar month from the lunation .
The Islamic calendar is based on the prohibition of intercalation (_nasi\' _) by Muhammad , in Islamic tradition dated to a sermon held on 9 Dhu al-Hijjah AH 10 (Julian date: 6 March 632). This resulted in an observationally based lunar calendar that shifts relative to the seasons of the solar year.
Main article: Calendar reform
The first calendar reform of the early modern era was the Gregorian calendar , introduced in 1582 based on the observation of a long-term shift between the Julian calendar and the solar year.
There have been a number of modern proposals for reform of the calendar, such as the World Calendar , International Fixed Calendar , Holocene calendar , and, recently, the Hanke-Henry Permanent Calendar . Such ideas are mooted from time to time but have failed to gain traction because of the loss of continuity, massive upheaval in implementation, and religious objections.
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 or Unix 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 * year and ordinal date within the year, e.g., the ISO 8601 ordinal date system
Calendars with two levels of cycles:
* year, month, and day – most systems, including the Gregorian calendar (and its very similar predecessor, the Julian calendar ), the Islamic calendar , the Solar Hijri calendar and the Hebrew calendar * year, week, and weekday – e.g., the ISO week date
Cycles can be synchronized with periodic phenomena: Sun and Moon , Schedel's Nuremberg Chronicle, 1493
* Lunar calendars are synchronized to the motion of the Moon (lunar phases ); an example is the Islamic calendar. * Solar calendars are based on perceived seasonal changes synchronized to the apparent motion of the Sun; an example is the Persian calendar . * Lunisolar calendars are based on a combination of both solar and lunar reckonings; examples include the traditional calendar of China , the Hindu calendar in India, and the Hebrew calendar. * 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.
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.
Main article: Solar calendar
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.
Main article: Lunar calendar
Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each lunar phase cycle. Because the length of the lunar month is not an even fraction of the length of the tropical year , a purely lunar calendar quickly drifts against the seasons, which don't vary much near the equator. It does, however, stay constant with respect to other phenomena, notably tides . An example is the Islamic calendar . Alexander Marshack, in a controversial reading, believed that marks on a bone baton (c. 25,000 BC) represented a lunar calendar. Other marked bones may also represent lunar calendars. Similarly, Michael Rappenglueck believes that marks on a 15,000-year-old cave painting represent a lunar 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 .
Nearly all calendar systems group consecutive days into "months" and also into "years". In a _solar calendar_ a _year_ approximates Earth's tropical year (that is, the time it takes for a complete cycle of seasons ), traditionally used to facilitate the planning of agricultural activities. In a _lunar calendar_, the _month_ approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week.
Because the number of days in the _tropical year_ is not a whole number, a solar calendar must have a different number of days in different years. This may be handled, for example, by adding an extra day in leap years . The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as intercalation . Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.
Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years. Many cultures use different baselines for their calendars' starting years. For example, the year in Japan is based on the reign of the current emperor: 2006 was Year 18 of the Emperor Akihito .
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
_ This section's FACTUAL ACCURACY IS DISPUTED . Relevant discussion may be found on Talk: Calendar . Please help to ensure that disputed statements are reliably sourced . (June 2017)_ _(Learn how and when to remove this template message )_
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.
CALENDARS IN USE
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 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, timekeepers can show time, date and weekday.
The Gregorian calendar is the _de facto _ international standard, and is used almost everywhere in the world for civil purposes. It is a purely solar calendar, with a cycle of leap days in a 400-year cycle designed to keep the duration of the year aligned with the solar year .
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). It was introduced in 1582 as a refinement to the Julian calendar which had been in use throughout the European Middle Ages, amounting to a 0.002% correction in the length of the year.
During the Early Modern period, however, its adoption was mostly limited to Roman Catholic nations, but by the 19th century, it became widely adopted worldwide for the sake of convenience in international trade. The last European country to adopt the reform was Greece, in 1923.
The calendar epoch used by the Gregorian calendar is the Dionysian era , inherited from the medieval convention associated with the Julian calendar. The year number is variously given as AD (for _Anno Domini _) or CE (for _ Common Era _ or, indeed, _Christian Era_).
_ A Hindu almanac (pancanga_) for the year 1871/2 from Rajasthan (Library of Congress, Asian Division)
The most important use of pre-modern calendars is keeping track of the liturgical year and the observation of religious feast days.
While the Gregorian calendar is itself historically motivated in relation to the calculation of the Easter date , it is now in worldwide secular use as the _de facto_ standard. Alongside the use of the Gregorian calendar for secular matters, there remain a number of calendars in use for religious purposes.
Eastern Christians , including the Orthodox Church , use the Julian calendar .
The Islamic calendar or Hijri calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in most of the Muslim countries (concurrently with the Gregorian calendar), and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals. Its epoch is the Hijra (corresponding to year 622 of the Dionysian era). With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately each 33 Islamic years.
Various Hindu calendars remain in use in the Indian subcontinent, including the Nepali calendar , Bengali calendar , Malayalam calendar , Tamil calendar , Vikrama Samvat used in Northern India, and Shalivahana calendar in the Deccan states.
Most of the Hindu calendars are inherited from a system first enunciated in Vedanga Jyotisha of Lagadha, standardized in the _Sūrya Siddhānta _ and subsequently reformed by astronomers such as Āryabhaṭa (499 CE), Varāhamihira (6th century) and Bhāskara II (12th century).
The Hebrew calendar is 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 cheques ).
Bahá\'ís worldwide use the Bahá\'í calendar .
The Chinese , Hebrew , Hindu , and Julian calendars are widely used for religious and social purposes.
The Iranian (Persian) calendar is used in Iran and some parts of Afghanistan . The Ethiopian calendar or Ethiopic calendar is the principal calendar used in Ethiopia and Eritrea , with the Oromo calendar also in use in some areas. In neighboring Somalia , the Somali calendar co-exists alongside the Gregorian and Islamic calendars. 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 .
Main article: Fiscal calendar _ The Payment of the Tithes_ (The tax-collector), also known as _Village Lawyer_, by Pieter Brueghel the Younger or workshop
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 on Diwali festival and end the day before the next year's Diwali festival.
In accounting (and particularly accounting software), a fiscal calendar (such as a 4/4/5 calendar ) fixes each month at a specific number of weeks to facilitate comparisons from month to month and year to year. January always has exactly 4 weeks (Sunday through Saturday), February has 4 weeks, March has 5 weeks, etc. Note that this calendar will normally need to add a 53rd week to every 5th or 6th year, which might be added to December or might not be, depending on how the organization uses those dates. There exists an international standard way to do this (the ISO week ). The ISO week starts on a Monday, and ends on a Sunday. Week 1 is always the week that contains 4 January in the Gregorian calendar.
A calendar from the Petaluma and Santa Rosa Railroad
The term _calendar_ applies not only to a given scheme of timekeeping but also to a specific record or device displaying such a scheme, for example an appointment book in the form of a pocket calendar (or personal organizer ), desktop calendar, a wall 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 USA 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. 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) and five to six rows (or rarely, four rows when the month of February contains 28 days 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.
Main article: Calendaring software Further information: Category:Calendaring standards
Calendaring software provides users with an electronic version of a calendar, and may additionally provide an appointment book, address book or contact list. Calendaring is a standard feature of many PDAs , EDAs , and smartphones . The software may be a local package designed for individual use (e.g., Lightning extension for Mozilla Thunderbird , Microsoft Outlook without Exchange Server, or Windows Calendar ) or may be a networked package that allows for the sharing of information between users (e.g., Mozilla Sunbird , Windows Live Calendar , Google Calendar , or Microsoft Outlook with Exchange Server).
* List of calendars * Advent calendar * Calendar reform * Calendrical calculation * Docket (court) * History of calendars * List of international common standards * List of unofficial observances by date * Real- Time Clock (RTC), which underlies the Calendar software on modern computers. * Unit of time
* ^ calendar 2017 . * ^ _New Shorter Oxford English Dictionary_ * ^ "Religion in the Etruscan period" in Roman religion in Encyclopædia Britannica * ^ Zerubavel 1985 . * ^ James Elkins, _Our beautiful, dry, and distant texts_ (1998) 63ff. * ^ "Oldest lunar calendar identified". BBC News. 2000-10-16. Retrieved 2013-03-14.
* "calendar", _American Heritage Dictionary_ (5th ed.), 2017 * Birashk, Ahmad (1993), _A comparative Calendar of the Iranian, Muslim Lunar, and Christian Eras for Three Thousand Years_, Mazda Publishers, ISBN 0-939214-95-4 * Björnsson, Árni (1995) , _High Days and Holidays in Iceland_, Reykjavík: Mál og menning, ISBN 9979-3-0802-8 , OCLC 186511596 * Dershowitz, Nachum ; Reingold, Edward M (1997), _Calendrical Calculations_, Cambridge University Press, ISBN 0-521-56474-3 , archived from the original on 17 October 2002 with Online Calculator * Doggett, LE (1992), "Calendars", in Seidelmann, P. Kenneth, _Explanatory Supplement to the Astronomical Almanac_, University Science Books, ISBN 0-935702-68-7 , archived from the original on 1 April 2004 * Richards, EG (1998), _Mapping Time, the calendar and its history_, Oxford University Press, ISBN 0-19-850413-6 * Rose, Lynn E (1999), _Sun, Moon, and Sothis_, Kronos Press, ISBN 0-917994-15-9 * Schuh, Dieter (1973), _Untersuchungen zur Geschichte der Tibetischen Kalenderrechnung_ (in German), Wiesbaden: Franz Steiner Verlag, OCLC 1150484 * Spier, Arthur (1986), _The Comprehensive Hebrew Calendar_, Feldheim Publishers, ISBN 0-87306-398-8 * Zerubavel, Eviatar (1985), _The Seven Day Circle: The History and Meaning of the Week_, University of Chicago Press, ISBN 0-226-98165-7
* Fraser, Julius Thomas (1987), _Time, the Familiar Stranger_ (illustrated ed.), Amherst: Univ of Massachusetts Press, ISBN 0-87023-576-1 , OCLC 15790499 * Whitrow, Gerald James (2003), _What is Time?_, Oxford: Oxford University Press, ISBN 0-19-860781-4 , OCLC 265440481 * C.K, Raju (2003), _The Eleven Pictures of Time_, SAGE Publications Pvt. Ltd, ISBN 0-76-199624-9 * C.K, Raju (1994), _Time: Towards a Consistent Theory_, Springer, ISBN 0-79-233103-6