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A calendar is a system of organizing days for social, religious, commercial or administrative purposes. This is done by giving names to periods of , typically , , and . A 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 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 or the . 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 over the long term.

The term calendar is taken from , 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. New Shorter Oxford English Dictionary 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 as calendier and from there in as calender by the 13th century (the spelling calendar is early modern).


History
The course of the Sun and the Moon are the most evident forms of , and the and 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."Religion in the Etruscan period" in Roman religion in Encyclopædia Britannica The first recorded calendars date to the , dependent on the development of writing in the Ancient Near East, the Egyptian and Sumerian calendars.

A large number of calendar systems which were based on the Babylonian calendar, and which were found in the Ancient Near East, date from the . Amongst such calendar systems was the calendar system of the Persian Empire, which in turn gave rise to the Zoroastrian calendar as well as the .

A great number of Hellenic calendars developed in , and with the Hellenistic period also influenced calendars outside of the immediate sphere of Greek influence, giving rise to the various as well as to the ancient .

Calendars in antiquity were , 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 .

The was reformed by 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 from the .

The is based on the prohibition of intercalation ( nasi') by , in Islamic tradition dated to a sermon held on 9 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.


Modern reforms
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 , 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.


Calendar systems
A full calendar system has a different calendar date for every day. Thus the week cycle is by itself not a full calendar system; neither is a system to name the days within a year without a system for identifying the years.

The simplest calendar system just counts time periods from a reference date. This applies for the or . 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 ), the , the Solar Hijri calendar and the
  • year, week, and weekday – e.g., the ISO week date

Cycles can be synchronized with periodic phenomena:

  • are synchronized to the motion of the Moon (); an example is the Islamic calendar.
  • are based on perceived changes synchronized to the apparent motion of the Sun; an example is the .
  • Lunisolar calendars are based on a combination of both solar and lunar reckonings; examples include the , the 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 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.


Solar calendars
Solar calendars assign a date to each . A day may consist of the period between and , with a following period of , 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 . Other types of calendar may also use a solar day.


Lunar calendars
Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each cycle. Because the length of the is not an even fraction of the length of the , 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 . An example is the . Alexander Marshack, in a controversial reading,James Elkins, Our beautiful, dry, and distant texts (1998) 63ff. 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.


Lunisolar calendars
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 .


Calendar subdivisions
Nearly all calendar systems group consecutive days into "months" and also into "years". In a solar calendar a year approximates Earth's tropical year (that is, the time it takes for a complete cycle of ), traditionally used to facilitate the planning of 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 . 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 is based on the reign of the current emperor: 2006 was Year 18 of the Emperor .


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 . Such a calendar is also referred to as an observation-based calendar. The advantage of such a calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult.

An arithmetic calendar is one that is based on a strict set of rules; an example is the current Jewish calendar. Such a calendar is also referred to as a rule-based calendar. The advantage of such a calendar is the ease of calculating when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation. This limits the lifetime of an accurate arithmetic calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar.


Complete and incomplete calendars
Calendars may be either complete or incomplete. Complete calendars provide a way of naming each consecutive day, while incomplete calendars do not. The early , 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 , 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 and calendar to help prevent them from overcommitting their time.

Calendars are also used as part of a complete system: date and time of day together specify a moment in . In the modern world, timekeepers can show time, date and weekday.


Gregorian calendar
The Gregorian calendar is the 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 .

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 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 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 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 ) or CE (for or, indeed, Christian Era).


Religious calendars
The most important use of pre-modern calendars is keeping track of the and the observation of religious feast days.

While the Gregorian calendar is itself historically motivated in relation to the calculation of the , 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 .

The or Hijri calendar, is a 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 (corresponding to AD 622) With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately each 33 Islamic years.

Various remain in use in the Indian subcontinent, including the , , Malayalam calendar, , used in Northern India, and calendar in the states.

The Buddhist calendar and the traditional lunisolar calendars of , , , and are also based on an older version of the Hindu calendar.

Most of the Hindu calendars are inherited from a system first enunciated in of Lagadha, standardized in the and subsequently reformed by astronomers such as (AD 499), (6th century) and Bhāskara II (12th century).

The is used by worldwide for religious and cultural affairs, also influences civil matters in Israel (such as ) and can be used there for business dealings (such as for the dating of ).

Bahá'ís worldwide use the Bahá'í calendar.


National calendars
The , , , and calendars are widely used for religious and social purposes.

The is used in and some parts of . The Ethiopian calendar or Ethiopic calendar is the principal calendar used in and , with the also in use in some areas. In neighboring , the co-exists alongside the Gregorian and Islamic calendars. In , 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 calendars

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 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.


Formats

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 in the form of a pocket calendar (or personal organizer), desktop calendar, a , 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.


Calendaring software
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 . The software may be a local package designed for individual use (e.g., Lightning extension for Mozilla Thunderbird, Microsoft Outlook without Exchange Server, or ) or may be a networked package that allows for the sharing of information between users (e.g., , Windows Live Calendar, , or Microsoft Outlook with Exchange Server).


See also
  • List of calendars
  • 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


Citations

Sources


Further reading

External links

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