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Do not fill this in! {{Short description|System for organizing the days of year}} {{About|the dating system|the physical object|Calendar (stationery)|other uses|Calendar (disambiguation)}} {{Redirect|📅|the unofficial holiday commonly represented by this emoji|World Emoji Day}} {{Distinguish|Calender}} {{Globalize|2=Eurasia|3=the Near East|date=September 2022|talk=Talk:Calendar#Systemic bias towards Eurasian calendars}} {{Use dmy dates|date=March 2021}} {{Use American English | date = November 2020}} {{Calendar|lk=on|show_year=on|float=right|month=current}} [[File:Kalender Indonesia.jpg|thumb|Modern day [[Indonesia]]n [[Gregorian calendar]] for a [[Catholic Church|Catholic]] [[Church (building)|church]]]] [[File:Calendar (part of a set) MET DP-13486-011.jpg|thumb|250px|British calendar, 1851, gilt bronze and [[malachite]], height: 20.3 cm, [[Metropolitan Museum of Art]] (New York City)]] A '''calendar''' is a system of organizing [[days]]. This is done by giving names to periods of [[time]], typically days, [[weeks]], [[month]]s and [[years]].<ref>{{Cite web |title=Calendars and their History |url=https://eclipse.gsfc.nasa.gov/SEhelp/calendars.html |access-date=2024-03-12 |website=eclipse.gsfc.nasa.gov}}</ref><ref>{{Cite book |last=Guo |first=Rongxing |url=https://books.google.com/books?id=8QNbDwAAQBAJ&newbks=0&printsec=frontcover&pg=PA159&dq=A+calendar+is+a+system+of+organizing+days.+This+is+done+by+giving+names+to+periods+of+time,+typically+days,+weeks,+months+and+years.&hl=vi |title=Human-Earth System Dynamics: Implications to Civilizations |date=2018-05-16 |publisher=Springer |isbn=978-981-13-0547-4 |pages=159 |language=en}}</ref><ref>{{Cite book |last=Bond |first=Thomas |url=https://books.google.com/books?id=ADJEAgAAQBAJ&newbks=0&printsec=frontcover&pg=RA1-PA9&dq=A+calendar+is+a+system+of+organizing+days.+This+is+done+by+giving+names+to+periods+of+time,+typically+days,+weeks,+months+and+years.&hl=vi |title=Singapore PSLE Mathematics Challenging Practice Solutions (Yellowreef) |last2=Hughes |first2=Chris |date=2013-12-03 |publisher=Yellowreef Limited |isbn=978-0-7978-0222-3 |language=en}}</ref> A [[calendar date|date]] 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 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, synchronized with the cycle of the [[solar calendar|sun]] or the [[lunar calendar|moon]].<ref>{{Cite web |date=2021-02-12 |title=Do menstrual and lunar cycles synchronize? What scientists say |url=https://www.medicalnewstoday.com/articles/menstrual-cycles-and-lunar-cycles-is-there-a-link |access-date=2024-03-12 |website=www.medicalnewstoday.com |language=en}}</ref><ref>{{Cite web |title=Introduction to Calendars |url=https://aa.usno.navy.mil/faq/calendars |access-date=2024-03-12 |website=aa.usno.navy.mil}}</ref><ref>{{Cite web |title=History – Ancient Egyptian Calendar |url=https://mabscross.co.uk/wp-content/uploads/2020/06/3.-History-Calendar.pdf |access-date=2024-03-12}}</ref> The most common type of pre-modern calendar was the [[lunisolar calendar]], a lunar calendar that occasionally adds one [[intercalary month]] to remain synchronized with the [[solar year]] over the long term. ==Etymology== The term ''calendar'' is taken from {{lang|la|[[calends|kalendae]]}},<ref>{{Cite book |last=Ma'Atnefert |first=Mwt Seshatms Nkatraet |url=https://books.google.com/books?id=C7KnAgAAQBAJ&newbks=0&printsec=frontcover&pg=PA9&dq=The+term+calendar+is+taken+from+kalendae,&hl=vi |title=You are Harmony ... Take Time to Harmonize ... Calendars and Time Connecting |date=2011 |publisher=Lulu.com |isbn=978-1-257-10758-2 |pages=9 |language=en}}</ref><ref>{{Cite book |last=Lewis |first=James R. |url=https://books.google.com/books?id=qQFlCwAAQBAJ&newbks=0&printsec=frontcover&pg=PA459&dq=The+term+calendar+is+taken+from+kalendae,&hl=vi |title=The Astrology Book: The Encyclopedia of Heavenly Influences |date=2003-03-01 |publisher=Visible Ink Press |isbn=978-1-57859-246-3 |pages=459 |language=en}}</ref> the term for the first day of the month in the Roman calendar, related to the verb {{lang|la|calare}} 'to call out', referring to the "calling" of the new moon when it was first seen.<ref>'' New Shorter Oxford English Dictionary''</ref> Latin {{lang|la|calendarium}} meant 'account book, register' (as accounts were settled and debts were collected on the calends of each month).<ref name=":0">{{Cite book |last=Sayeed |first=Ahmed |url=https://books.google.com/books?id=GVeoDwAAQBAJ&newbks=0&printsec=frontcover&pg=PT408&dq=Latin+calendarium+meant+'account+book,+register'&hl=vi |title=You Must Win: The winner can create History |date=2019-08-10 |publisher=Prowess Publishing |isbn=978-1-5457-4730-8 |language=en}}</ref> The Latin term was adopted in [[Old French]] as {{lang|fro|calendier}} and from there in [[Middle English]] as {{lang|enm|calender}} by the 13th century (the spelling ''calendar'' is early modern).<ref name=":0" /> == History == {{Main|History of calendars}} {{Further|Week|Calendar epoch|Month|Lunisolar calendar|Computus|Calendar reform}} [[File:Equinozio da Pizzo Vento,tramonto fondachelli fantina, sicilia.JPG|thumb|[[Equinox]] seen from the astronomic calendar of Pizzo Vento at [[Fondachelli-Fantina|Fondachelli Fantina]], [[Sicily]]]] The course of the Sun and the Moon are the most salient regularly recurring natural events useful for [[timekeeping]], and in pre-modern societies around the world [[lunation]] and the [[year]] were most commonly used as time units. Nevertheless, the Roman calendar contained remnants of a very ancient pre-Etruscan 10-month solar year.<ref>"Religion in the Etruscan Period" in [http://global.britannica.com/topic/Roman-religion Roman Religion] {{Webarchive|url=https://web.archive.org/web/20151015213053/http://global.britannica.com/topic/Roman-religion |date=15 October 2015 }} in the ''Encyclopædia Britannica''</ref> The first recorded physical calendars, dependent on the development of [[Bronze Age writing|writing]] in the [[Ancient Near East]], are the [[Bronze Age]] [[Egyptian calendar|Egyptian]] and [[Sumerian calendar|Sumerian]] calendars.<ref>{{Cite journal|last=Shirley|first=Lawrence|date=11 February 2009|title=The Mayan and Other Ancient Calendars|url=http://dx.doi.org/10.4169/loci003264|location=Washington, DC|doi=10.4169/loci003264|journal=Convergence|access-date=16 March 2021|archive-date=9 April 2023|archive-url=https://web.archive.org/web/20230409163211/https://www.maa.org/press/periodicals/convergence/the-mayan-and-other-ancient-calendars|url-status=live}}</ref> During the [[Vedic period]] India developed a sophisticated timekeeping methodology and calendars for [[Vedas|Vedic]] rituals.<ref>{{Cite book|last=Plofker|first=Kim|title=Mathematics in India|publisher=Princeton University Press|year=2009|isbn=978-0-691-12067-6|pages=10, 35-36, 67}}</ref> 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.<ref>{{Cite book|last=Andersen|first=Johannes|url=https://books.google.com/books?id=gQYscrT0fgQC|title=Highlights of Astronomy, Volume 11B: As Presented at the XXIIIrd General Assembly of the IAU, 1997|date=31 January 1999|publisher=Springer Science & Business Media|isbn=978-0-7923-5556-4|pages=719|language=en|access-date=12 March 2021|archive-date=11 January 2023|archive-url=https://web.archive.org/web/20230111163620/https://books.google.com/books?id=gQYscrT0fgQC|url-status=live}}</ref> A large number of calendar systems in the Ancient Near East were based on the [[Babylonian calendar]] dating from the [[Iron Age]],<ref>{{Cite book |url=https://books.google.com/books?id=cg0OAQAAMAAJ&newbks=0&printsec=frontcover&dq=A+large+number+of+calendar+systems+in+the+Ancient+Near+East+were+based+on+the+Babylonian+calendar+dating+from+the+Iron+Age&q=A+large+number+of+calendar+systems+in+the+Ancient+Near+East+were+based+on+the+Babylonian+calendar+dating+from+the+Iron+Age&hl=vi |title=Civilizations of the Ancient Near East |date=1995 |publisher=Scribner |isbn=978-0-684-19279-6 |language=en}}</ref> among them the calendar system of the Persian Empire, which in turn gave rise to the [[Zoroastrian calendar]] and the [[Hebrew calendar]].<ref>{{Cite web |date=2021-09-12 |title=Egyptians celebrate new Egyptian year on September 11 |url=https://www.egypttoday.com/Article/4/107766/Egyptians-celebrate-new-Egyptian-year-on-September-11 |access-date=2024-03-12 |website=EgyptToday}}</ref><ref>{{Cite book |last=Sayeed |first=Ahmed |url=https://books.google.com/books?id=GVeoDwAAQBAJ&newbks=0&printsec=frontcover&pg=PT407&dq=calendar+system+of+the+Persian+Empire,+which+in+turn+gave+rise+to+the+Zoroastrian+calendar+and+the+Hebrew+calendar.&hl=vi |title=You Must Win: The winner can create History |date=2019-08-10 |publisher=Prowess Publishing |isbn=978-1-5457-4730-8 |language=en}}</ref> A great number of [[Hellenic calendars]] were developed in [[Classical Greece]], and during the [[Hellenistic period]] they gave rise to the ancient [[Roman calendar]] and to various [[Hindu calendars]].<ref>{{Cite web|title=Calendar - The Early Roman Calendar|url=https://www.britannica.com/science/calendar|access-date=19 January 2021|date=24 December 2020|website=Encyclopedia Britannica|language=en|archive-date=4 January 2021|archive-url=https://web.archive.org/web/20210104181246/https://www.britannica.com/science/calendar|url-status=live}}</ref> Calendars in antiquity were [[lunisolar]], depending on the introduction of [[intercalary month]]s 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 46 BC.<ref>{{cite web|title=The History of the Calendar|url=https://www.calendar.com/history-of-the-calendar/ |archive-url=https://web.archive.org/web/20201202131025/https://www.calendar.com/history-of-the-calendar/ |archive-date=2020-12-02 |url-status=dead}}</ref> 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 calendar [[month]] from [[lunation]]. The [[Gregorian calendar]], introduced in 1582, corrected most of the remaining difference between the Julian calendar and the solar year. The [[Islamic calendar]] is based on the prohibition of intercalation (''[[nasi']]'') by [[Muhammad]], in Islamic tradition dated to a sermon given on 9 [[Dhu 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 the [[World Calendar]], the [[International Fixed Calendar]], the [[Holocene calendar]], and 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 and the massive upheaval that implementing them would involve, as well as their effect on cycles of religious activity. == Systems == [[Image:Universal Calendar including a meton cycle.svg|350px|thumb|A universal calendar, combining different calendars]] A full calendar system has a different calendar date for every day.<ref>{{Cite book |last=Onlineverdan |url=https://books.google.com/books?id=f3uMDwAAQBAJ&newbks=0&printsec=frontcover&pg=SA10-PA6&dq=A+full+calendar+system+has+a+different+calendar+date+for+every+day&hl=vi |title=APTITUDE & REASONING for GATE & ESE 2020 |publisher=Infinity Educations |isbn=978-81-940294-3-4 |pages=10-6 |language=en}}</ref><ref>{{Cite book |last=Jain |first=Hemant |url=https://books.google.com/books?id=G4yEDwAAQBAJ&newbks=0&printsec=frontcover&pg=SA8-PA5&dq=A+full+calendar+system+has+a+different+calendar+date+for+every+day&hl=vi |title=RRB Junior Engineer (2019) - MATHEMATICS for 1st STAGE CBT |date=2019-01-21 |publisher=Infinity Educations |isbn=978-81-939356-9-9 |pages=8-5 |language=en}}</ref> Thus the week cycle is by itself not a full calendar system;<ref>{{Cite book |last=Jain |first=Hemant |url=https://books.google.com/books?id=G4yEDwAAQBAJ&newbks=0&printsec=frontcover&pg=SA8-PA5&dq=Thus+the+week+cycle+is+by+itself+not+a+full+calendar+system&hl=vi |title=RRB Junior Engineer (2019) - MATHEMATICS for 1st STAGE CBT |date=2019-01-21 |publisher=Infinity Educations |isbn=978-81-939356-9-9 |pages=8-5 |language=en}}</ref> 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.<ref>{{Cite book |last=Jain |first=Hemant |url=https://books.google.com/books?id=G4yEDwAAQBAJ&newbks=0&printsec=frontcover&pg=SA8-PA6&dq=The+simplest+calendar+system+just+counts+time+periods+from+a+reference+date.&hl=vi |title=RRB Junior Engineer (2019) - MATHEMATICS for 1st STAGE CBT |date=2019-01-21 |publisher=Infinity Educations |isbn=978-81-939356-9-9 |pages=8-6 |language=en}}</ref> 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 dates|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: [[File:Sun and Moon Nuremberg chronicle.jpg|250px|thumbnail|right|[[Sun]] and [[Moon]], Schedel's Nuremberg Chronicle, 1493]] * [[Lunar calendar]]s are synchronized to the motion of the Moon ([[lunar phase]]s); an example is the Islamic calendar. * [[Solar calendar]]s are based on perceived [[seasonal year|seasonal]] changes synchronized to the apparent motion of the Sun; an example is the [[Persian calendar]]. * [[Lunisolar calendar]]s are based on a combination of both solar and lunar reckonings; examples include the [[Chinese calendar|traditional calendar of China]], the [[Hindu calendar]] in India and Nepal, 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 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.{{sfn|Zerubavel | 1985}} ===Solar=== {{main|Solar calendar}} Solar calendars assign a ''date'' to each [[solar time|solar day]].<ref>{{Cite web |title=Introduction to Calendars |url=https://aa.usno.navy.mil/faq/calendars |access-date=2024-03-12 |website=aa.usno.navy.mil}}</ref> 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.<ref>{{Cite book |url=https://books.google.com/books?id=drMzAQAAMAAJ&newbks=0&printsec=frontcover&pg=PA983&dq=A+day+may+consist+of+the+period+between+sunrise+and+sunset,+with+a+following+period+of+night&hl=vi |title=The Jurist |date=1861 |publisher=S. Sweet |pages=983 |language=en}}</ref><ref>{{Cite book |last=Oxossi |first=Diego de |url=https://books.google.com/books?id=FN11EAAAQBAJ&newbks=0&printsec=frontcover&pg=PT37&dq=A+day+may+consist+of+the+period+between+sunrise+and+sunset,+with+a+following+period+of+night&hl=vi |title=Sacred Leaves: A Magical Guide to Orisha Herbal Witchcraft |date=2022-07-08 |publisher=Llewellyn Worldwide |isbn=978-0-7387-6721-5 |language=en}}</ref> 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 [[solar time|mean 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,<ref>{{Cite web |title=Solar calendar {{!}} Ancient Egypt, Mayan, Aztec {{!}} Britannica |url=https://www.britannica.com/science/solar-calendar |access-date=2024-03-12 |website=www.britannica.com |language=en}}</ref> 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.<ref>{{Cite book |url=https://books.google.com/books?id=mXsoAQAAIAAJ&newbks=0&printsec=frontcover&dq=The+Egyptians+appear+to+have+been+the+first+to+develop+a+solar+calendar,&q=The+Egyptians+appear+to+have+been+the+first+to+develop+a+solar+calendar,&hl=vi |title=The New Encyclopædia Britannica: Micropædia |date=1991 |publisher=Encyclopædia Britannica |isbn=978-0-85229-529-8 |pages=941 |language=en}}</ref><ref>{{Cite book |last=Lawson |first=Russell M. |url=https://books.google.com/books?id=kwTHEAAAQBAJ&newbks=0&printsec=frontcover&pg=PT179&dq=The+Egyptians+appear+to+have+been+the+first+to+develop+a+solar+calendar,&hl=vi |title=Science in the Ancient World: From Antiquity through the Middle Ages |date=2021-09-23 |publisher=Bloomsbury Publishing USA |isbn=979-8-216-14241-6 |language=en}}</ref><ref>{{Cite book |last=Muntz |first=Charles |url=https://books.google.com/books?id=VGHODQAAQBAJ&newbks=0&printsec=frontcover&pg=PT16&dq=The+Egyptians+appear+to+have+been+the+first+to+develop+a+solar+calendar,&hl=vi |title=Diodorus Siculus and the World of the Late Roman Republic |date=2017-01-02 |publisher=Oxford University Press |isbn=978-0-19-064901-2 |language=en}}</ref> 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 didn't include the extra bit of time in each year, and this caused their calendar to slowly become inaccurate.<ref>{{Cite web |title=Solar calendar | chronology | Britannica |url=https://www.britannica.com/science/solar-calendar}}</ref> ===Lunar=== {{Main|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 do not vary much near the equator. It does, however, stay constant with respect to other phenomena, notably [[tide]]s. An example is the [[Islamic calendar]]. Alexander Marshack, in a controversial reading,<ref>James Elkins, ''[https://books.google.com/books?id=5Ku6YdWurMgC&pg=PA63&lpg=PA63 Our beautiful, dry, and distant texts]'' (1998) 63ff.</ref> believed that marks on a bone baton ({{Circa|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.<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/975360.stm |title=Oldest lunar calendar identified |work=BBC News |date=16 October 2000 |access-date=14 March 2013 |archive-date=11 February 2021 |archive-url=https://web.archive.org/web/20210211233146/http://news.bbc.co.uk/2/hi/science/nature/975360.stm |url-status=live }}</ref> ===Lunisolar=== A [[lunisolar calendar]] is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons. Prominent examples of lunisolar calendar are [[Hindu calendar]] and [[Buddhist calendar]] that are popular in [[South Asia]] and [[Southeast Asia]]. Another example is the Hebrew calendar, which uses a [[metonic cycle|19-year cycle]]. ==Subdivisions== {{see also|Decade|Century|Millennium}} [[File:Perpetual calendar in Catalan circa year 1925 Patent in Barcelona.jpg|left|thumb|Catalan early 20th century perpetual calendar]] 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 [[season]]s), traditionally used to facilitate the planning of [[agriculture|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 year]]s. 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 (timekeeping)|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. Historically, several countries have based their calendars on [[regnal years]], a calendar based on the reign of their current sovereign. For example, the year 2006 in [[Japan]] is year 18 Heisei, with Heisei being the era name of [[Emperor of Japan|Emperor]] [[Akihito]]. ==Other types== {{anchor|Observational}}<!--linked--> ===Arithmetical and astronomical=== [[File:JudischerKalender-1831 ubt.jpeg|thumb|Calendar of the [[Qahal]], 5591 (1831)]] 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. ===Other variants=== The early [[Roman calendar]], created during the reign of [[Romulus]], lumped the 61 days of the winter period them together as simply "winter."<ref>{{Cite web |last=Jones |first=Derek |date=2018-03-08 |title=Roman Calendar |url=https://editions.covecollective.org/chronologies/roman-calendar |access-date=2024-03-18 |website=editions.covecollective.org |language=en}}</ref> Over time, this period became January and February; through further changes over time (including the creation of the [[Julian calendar]]) this calendar became the modern Gregorian calendar, introduced in the 1570s.<ref>{{Cite web |date=2023-12-29 |title=Who Decided January 1st Is the New Year? |url=https://time.com/6550127/new-year-celebration-january-calendar-date-history/ |access-date=2024-03-18 |website=TIME |language=en}}</ref><ref>{{Cite web |last=Kelechava |first=Brad |date=2016-02-11 |title=History of the Standard Gregorian Calendar |url=https://blog.ansi.org/2016/02/history-of-standard-gregorian-calendar/ |access-date=2024-03-18 |website=The ANSI Blog |language=en-US}}</ref> ==Usage== [[File:World calendars map.svg|thumb|center|800px|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 the [[Chinese calendar|Chinese]] or [[Islamic calendar|Islamic]] 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 are [[religious holiday|religious]] or [[civil holiday]]s, 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 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. Some may also show the lunar phase. ===Gregorian=== The [[Gregorian calendar]] is the ''[[de 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 the [[solar year]].{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 682–683}} There is a lunar aspect which approximates the position of the moon during the year, and is used in the [[Computus | calculation of the date of Easter]].{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 817–820}} 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).{{sfn | Dershowitz | Reingold | 2008 | pp = 47, 187}} The calendar 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.{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 682–683}} During the Early Modern period, [[Adoption of the Gregorian calendar|its adoption was mostly limited]] to [[Roman 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.{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 682–689}} The [[calendar epoch]] used by the Gregorian calendar is inherited from the medieval convention established by [[Dionysius Exiguus]] and associated with the Julian calendar. The year number is variously given as AD (for ''[[Anno Domini]]'') or CE (for ''[[Common Era]]'' or ''Christian Era'').{{sfn | Blackburn | Holford-Strevens | 2003 | loc= Chapter: "Christian Chronology"}} ===Religious=== [[File:Hindu calendar 1871-72.jpg|thumb|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 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 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 of [[Advent]], [[Christmas]], Ordinary Time (Time after [[Epiphany (holiday)|Epiphany]]), [[Lent]], [[Easter]], and Ordinary Time (Time after [[Pentecost]]). Some Christian calendars do not include Ordinary Time and every day falls into a denominated season. [[Eastern Christians]], including the [[Eastern Orthodox Church|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 [[Anno Hegirae|Hijra]] (corresponding to AD 622). With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately every 33 Islamic years. Various [[Hindu calendar]]s remain in use in the Indian subcontinent, including the [[Nepali calendar (disambiguation)|Nepali calendars]], [[Bengali calendar]], [[Malayalam calendar]], [[Tamil calendar]], [[Vikrama Samvat]] used in Northern India, and [[Shalivahana era|Shalivahana]] calendar in the [[Deccan Plateau|Deccan]] states. The [[Buddhist calendar]] and the traditional lunisolar calendars of [[Cambodia]], [[Laos]], [[Myanmar]], [[Sri Lanka]] and [[Thailand]] are also based on an older version of the Hindu calendar. Most of the Hindu calendars are inherited from a system first enunciated in [[Vedanga Jyotisha]] of Lagadha, standardized in the ''[[Surya Siddhanta|Sūrya Siddhānta]]'' and subsequently reformed by astronomers such as [[Aryabhata|Āryabhaṭa]] (AD 499), [[Varahamihira|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 [[public holiday|national holidays]]) and can be used business dealings (such as for the dating of [[cheque]]s).<ref>{{Cite web |title=About the Hebrew Calendar {{!}} Yale University Library |url=https://web.library.yale.edu/cataloging/hebraica/about-hebrew-calendar |access-date=2024-03-18 |website=web.library.yale.edu}}</ref> Followers of the [[Baháʼí Faith]] use the [[Baháʼí 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. ===National=== The [[Chinese calendar|Chinese]], [[Hebrew calendar|Hebrew]], [[Hindu calendar|Hindu]], and [[Julian calendar|Julian]] calendars are widely used for religious and social purposes. The [[Solar Hijri calendar|Iranian (Persian) calendar]] is used in [[Iran]] and some parts of [[Afghanistan]]. The [[Assyrian 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. 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]]. ===Fiscal=== {{Main|Fiscal calendar}} [[File:Pieter Brueghel the Younger (or workshop) The Payment of the Tithes Bonhams.jpg|thumb|''The Payment of the Tithes'' (The tax-collector), also known as ''Village Lawyer'', by [[Pieter Brueghel the Younger]] ]] 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|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 8601|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. ==Formats== {{Unreferenced section|date=April 2023}} [[File:Petaluma and Santa Rosa Railroad Co. Calendar.jpg|thumb|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 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.{{cn|date=April 2023}} 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){{cn|date=April 2023}} and five to six rows (or rarely, four rows when the month of February contains 28 days in [[common year]]s 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. ==Software== {{Main|Calendaring software}} {{Further|: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 [[Personal digital assistant|PDAs]], [[Enterprise digital assistant|EDAs]], and [[smartphones]]. The software may be a local package designed for individual use (e.g., [[Lightning (software)|Lightning]] extension for [[Mozilla Thunderbird]], [[Microsoft Outlook]] without Exchange Server, or [[Windows Calendar]]) or maybe 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). == See also == {{div col}} * [[General Roman Calendar]] * [[List of calendars]] * [[Advent calendar]] * [[Calendar reform]] * [[Calendrical calculation]] * [[Docket (court)]] * [[History of calendars]] * [[Horology]] * [[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]] {{div col end}} == References == === Citations === {{Reflist}} === Sources === * {{citation|article = calendar|title = American Heritage Dictionary|date = 2017|edition = 5th|ref={{harvid|calendar|2017}}|url = https://www.ahdictionary.com/word/search.html?q=calendar}} * {{citation|title=A Comparative Calendar of the Iranian, Muslim Lunar, and Christian Eras for Three Thousand Years|first=Ahmad|last=Birashk|publisher=Mazda Publishers|year=1993|isbn=978-0-939214-95-2}} * {{citation|title=High Days and Holidays in Iceland|first=Árni|last = Björnsson|location=Reykjavík|publisher=Mál og menning|year=1995|isbn=978-9979-3-0802-7|oclc=186511596|orig-year=1977}} * {{citation|last1 = Blackburn |first1 = Bonnie | last2 = Holford-Strevens | first2 = Leofranc | title = The Oxford Companion to the Year | date = 2003 | orig-year = 1999 | edition = corrected reprinting of 1st | publisher = Oxford University Press}} * {{citation|title=Calendrical Calculations|title-link= Calendrical Calculations |first1=Nachum|last1=Dershowitz|author1-link=Nachum Dershowitz|first2=Edward M|last2=Reingold|author2-link=Edward M. Reingold|publisher=Cambridge University Press|year=2008 |isbn=978-0-521-70238-6}} * {{citation|title=Explanatory Supplement to the Astronomical Almanac|editor-first=P. Kenneth|editor-last=Seidelmann|publisher=University Science Books|year=1992|chapter=Calendars|chapter-url=https://archive.org/details/131123ExplanatorySupplementAstronomicalAlmanac/page/n301/mode/1up|first=L.E.|last=Doggett|isbn=978-0-935702-68-2}} * {{citation|title=Mapping Time, the calendar and its history|first=E.G.|last=Richards|publisher=Oxford University Press|year=1998|isbn=978-0-19-850413-9|url-access=registration|url=https://archive.org/details/mappingtimecalen00rich}} * {{citation|title=Sun, Moon, and Sothis|first=Lynn E.|last=Rose|publisher=Kronos Press|year=1999|isbn=978-0-917994-15-9}} * {{citation|title=Untersuchungen zur Geschichte der Tibetischen Kalenderrechnung|first=Dieter|last =Schuh|location=Wiesbaden|publisher=Franz Steiner Verlag|year=1973|oclc=1150484|language=de}} * {{citation|title=The Comprehensive Hebrew Calendar|first=Arthur|last=Spier|publisher=Feldheim Publishers|year=1986|isbn=978-0-87306-398-2}} * {{citation|first=Eviatar|last=Zerubavel|title=The Seven Day Circle: The History and Meaning of the Week|publisher=University of Chicago Press|year=1985|isbn=978-0-226-98165-9}} == Further reading == * {{citation|title=Time, the Familiar Stranger|last=Fraser|first=Julius Thomas|edition=illustrated|location=Amherst|publisher=Univ of Massachusetts Press|year=1987|isbn=978-0-87023-576-4|oclc=15790499|bibcode=1988tfs..book.....F}} * {{citation|title=What is Time?|last=Whitrow|first=Gerald James|location=Oxford|publisher=Oxford University Press|year=2003|isbn=978-0-19-860781-6|oclc=265440481}} * {{citation|title=The Eleven Pictures of Time|last=C.K|first=Raju|publisher=SAGE Publications Pvt. Ltd|year=2003|isbn=978-0-7619-9624-8}} * {{citation|title=Time: Towards a Consistent Theory|last=C.K|first=Raju|publisher=Springer|year=1994|isbn=978-0-7923-3103-2}} ==External links== {{Commons category|Calendars}} {{Wiktionary|calendar}} * {{cite EB9 |wstitle = Calendar |volume= IV | pages=664-682 |short=1}} * {{cite EB1911|wstitle=Calendar|volume=4|page=987-1004|short=1}} * {{cite Americana|wstitle=Calendar|short=1}} *[http://www.fourmilab.ch/documents/calendar/ Calendar converter, including all major civil, religious and technical calendars.] <!-- PLEASE do not link to information or examples of any other specific calendars on this page. This article is about calendars in general. --> {{calendars}} {{time topics}} {{time measurement and standards}} {{chronology}} {{Authority control}} [[Category:Calendars| ]] [[Category:Units of time]] Summary: Please note that all contributions to Christianpedia may be edited, altered, or removed by other contributors. 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