A calendar is a system for naming periods of time, typically days. These names are known as calendar dates. Cycles in a calendar are often synchronised with the perceived motion of astronomical objects.
A calendar is also a physical device (often paper). This is the most common usage of the word.
As a subset, 'calendar' is also used to denote a list of particular set of planned events (for example, court calendar).
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 days from a reference day. This applies for the Julian day. Virtually the only possible variation is using a different reference day, 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, more commonly, 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, and the Hebrew calendar
- year, week, and weekday - e.g. the ISO week date
Cycles can be synchronised with periodic phenomena:
- A lunar calendar is synchronized to the motion of the Moon (lunar phases); an example is the Islamic calendar.
- A solar calendar is based on perceived seasonal changes synchronized to the apparent motion of the Sun; an example is the Persian calendar.
- There are some calendars that appear to be synchronized to the motion of Venus, such as some of the ancient Egyptian calendars; synchronization to Venus appears to occur primarily in civilizations near the Equator.
- The week cycle is an example of one that is not synchronized to any external phenomenon.
Very commonly a calendar includes more than one type of cycle, or has both cyclic and acyclic elements. A lunisolar calendar is synchronized both to the motion of the Moon and to the apparent motion of the Sun; an example is the Jewish calendar.
Many calendars incorporate simpler calendars as elements. For example, the rules of the Jewish calendar depend on the seven-day week cycle (a very simple calendar), so the week is one of the cycles of the Jewish calendar. It is also common to operate two calendars simultaneously, usually providing unrelated cycles, and the result may also be considered a more complex calendar. For example, the Gregorian calendar has no inherent dependence on the seven-day week, but in Western society the two are used together, and calendar tools indicate both the Gregorian date and the day of week.
The week cycle is shared by various calendar systems (although the significance of special days such as Friday, Saturday, and Sunday varies). Systems of leap days usually do not affect the week cycle. The week cycle was not even interrupted when 10 dates were skipped after the last day of the Julian calendar Thursday October 4, 1582, which was followed by the first day of the Gregorian calendar, Friday October 15, 1582, when the Gregorian calendar was first introduced on February 24, 1582.
Days used by solar calendars
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.
There have been a number of proposals for reform of the calendar, such as the World calendar, International Fixed Calendar, Holocene calendar, and Sol Calendar. The United Nations considered adopting such a reformed calendar for a while in the 1950s, but these proposals have lost most of their popularity.
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. It does, however, stay constant with respect to other phenomena, notably tides. 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 Jewish calendar which uses a 19 year cycle..
Lunar calendars are believed to be the oldest calendars invented by mankind. Cro-Magnon people are claimed to have invented one around 32,000 BC.
A fiscal calendar (such as a 5/4/4 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 5 weeks (Sunday through Saturday), February has 4 weeks, March has 4 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 runs Monday through Sunday and Week 1 is always the week that contains January 4 Gregorian.
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 (29 February) 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 would be Year 18 of the Emperor Akihito.
Other calendar types
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 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.
Pragmatic, theoretical and mixed calendars
Calendars may be pragmatic, theoretical, or mixed.
A pragmatic calendar is based on 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 or astronomical 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.
A theoretical 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 a rule-based or arithmetical calendar. The advantage of such a calendar is the ease of working out when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore, even if the calendar is very accurate, its accuracy perishes slowly over time owing to changes in Earth's rotation. This limits the lifetime of an accurate theoretical calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar, resulting in a mixed calendar.
A mixed calendar combines the features of both pragmatic and theoretical calendars. Mixed calendars usually begin as theoretical calendars, but are adjusted pragmatically when some type of asynchrony becomes apparent; the shift from the Julian to the Gregorian calendar is such an example.
The Gregorian calendar, as a final example, is complete, solar, and mixed.
The primary practical use of a calendar is to identify days: to be informed about and/or to agree on a future event and to record an event that has happened. Days may be significant for civil, religious or social reasons. For example, a calendar provides a way to determine 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 as part of a complete timekeeping system: date and time of day together specify a moment in time. In the modern world, written calendars are no longer an essential part of such systems, as the advent of accurate clocks has made it possible to record time independently of astronomical events.
Currently used calendars
Calendars in widespread use today include the Gregorian calendar, which is the de facto international standard, and is used almost everywhere in the world for civil purposes, including in the People's Republic of China and India (along with the Indian national calendar). Due to the Gregorian calendar's obvious connotations of Christianity, non-Christians sometimes justify its use by replacing the traditional era notations "AD" and "BC" ("Anno Domini" and "Before Christ") with "CE" and "BCE" ("Common Era" and "Before Common Era"). The Hindu calendars are some of the most ancient calendars of the world. The Gregorian calendar is widely used in Israel's business and day-to-day affairs, but the Hebrew calendar is used for religious affairs.
Also, the Persian calendar is used in Iran and Afghanistan. The Islamic calendar is used by the non-Persian Muslims the world over. The Chinese, Hebrew, Hindu, and Julian calendars are widely used for religious and/or social purposes. The Ethiopian calendar or Ethiopic calendar is the principal calendar used in Ethiopia and Eritrea.
Even where there is a commonly used calendar such as the Gregorian calendar, alternate calendars may also be used, such as a fiscal calendar or the astronomical year numbering system.
A calendar is also a physical device (often paper) (for example, a desktop calendar); one sheet 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 show what the current date and weekday is. This is the most common usage of the word.
The sale of physical calendars has been restricted in some countries, and given as a monopoly to universities and national academies. Examples include the Prussian Academy of Sciences and the University of Helsinki, which had a monopoly on the sale of calendars in Finland until the 1990s.
- Active calendar
- Anno Domini Christian Era
- Aztec Calendar
- Bahá'í calendar
- Bengali calendar
- Calculating the day of the week
- Calendar of saints
- Calendar reform
- Christian calendar
- Common era
- Discordian calendar
- Eastern Orthodox liturgical calendar
- Ethiopian calendar
- French Republican Calendar
- Gregorian calendar
- Hebrew calendar
- Hindu calendar
- Iranian calendar
- Islamic calendar
- Julian calendar
- List of calendars
- Liturgical year
- Maya calendar
- Perpetual calendar
- Runic calendar
- Wall calendar
- Zoroastrian calendar
- Calendrical Calculations; Nachum Dershowitz and Edward M. Reingold; Cambridge University Press, 1997; ISBN 0-521-56474-3; Book Info; Online Calculator
- Mapping Time, the calendar and its history; E G Richards; Oxford University Press, 1998; ISBN 0-19-850413-6
- A comparative Calendar of the Iranian, Muslim Lunar, and Christian Eras for Three Thousand Years; Ahmad Birashk; Mazda Publishers, 1993; ISBN 0-939214-95-4
- The Comprehensive Hebrew Calendar; Arthur Spier; Feldheim Publishers, 1986; ISBN 0-87306-398-8
- High Days and Holidays in Iceland; Árni Björnsson; Mál og menning, 1995; ISBN 9979-3-0802-8
- Explanatory Supplement to the Astronomical Almanac; P. Kenneth Seidelmann, ed.; University Science Books, 1992; ISBN 0-935702-68-7; Chapter 12: Calendars by L. E. Doggett
- Sun, Moon, and Sothis; Lynn E. Rose; Kronos Press, 1999; ISBN 0-917994-15-9
- Dieter Schuh (1973). Untersuchungen zur Geschichte der Tibetischen Kalenderrechnung. Wiesbaden: Franz Steiner Verlag GMBH. OCLC 1150484.
- ^ NASA - Year Dating Conventions
- Frequently Asked Questions about Calendars
- 1911 Encyclopaedia Britannica entry
- Various calendars described as part of the Calendars through the Ages online exhibit
- Perpetual Calendar 1800 - 2400
- Perpetual Calendar
- Current calendar.
- DateDex: Selected events on selected dates
- Ancient Calendars NIST website
- Yearly perpetual calendar
- Historic Events
- Date calculator
- The Ten Thousand Year Calendar - Western Chinese Calendar Converter
- The Chinese Calendars
- English Date to Tamil Date Conversion
- Dynamic Tamil Calendar