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A calendar is a system of organizing days. This is done by giving names to periods of
time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

time
, typically
day The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems. As a term in physics and astronomy it is approximately the period during which the Earth completes one ro ...

day
s,
week A week is a time unit equal to seven days. It is the standard time period used for cycles of rest days in most parts of the world, mostly alongside—although not strictly part of—the Gregorian calendar. In many languages, the days of the we ...

week
s,
month A month is a unit of time, used with calendars, that is approximately as long as a natural orbital period of the Moon; the words ''month'' and ''Moon'' are cognates. The traditional concept arose with the cycle of Moon phases; such lunar months ("l ...
s and
year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the ...
s. 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 A docket in the United States is the official summary of proceedings in a Court, court of law. In the United Kingdom in modern times it is an official document relating to delivery of something, with similar meanings to these two elsewhere. In th ...
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
sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...
or the
moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...
. The most common type of pre-modern calendar was the
lunisolar calendar A lunisolar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day ...
, a lunar calendar that occasionally adds one
intercalary month Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendar A lunisolar calendar is a calendar in many culture Cul ...
to remain synchronized with the
solar year A tropical year (also known as a solar year or tropical period) is the time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past ...
over the long term.


Etymology

The term ''calendar'' is taken from ''
calendaeThe calends or kalends ( la, kalendae) is the first day of every month in the Roman calendar. The English language, English word ''calendar'' is derived from this word. Use The Ancient Rome, Romans called the first day of every month the ''calends ...
'', 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 Old French (, , ; Modern French French ( or ) is a Romance language The Romance languages, less commonly Latin or Neo-Latin languages, are the modern languages that evolved from Vulgar Latin Vulgar Latin, also known as Popular o ...
as ''calendier'' and from there in
Middle English Middle English (abbreviated to ME) was a form of the English language English is a West Germanic language of the Indo-European language family The Indo-European languages are a language family A language is a structured sys ...
as ''calender'' by the 13th century (the spelling ''calendar'' is early modern).


History

The course of the sun and the moon are the most salient natural, regularly recurring events useful for
timekeeping Time is the indefinite continued progress of existence Existence is the ability of an entity to interact with physical or mental reality Reality is the sum or aggregate of all that is real or existent within a system, as opposed to that wh ...
, thus in pre-modern societies worldwide
lunation In lunar calendar A lunar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a sing ...
and the
year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the ...
were most commonly used as time units. Nevertheless, the Roman calendar contained remnants of a very ancient pre-Etruscan 10-month solar year. The first recorded physical calendars, dependent on the development of
writing Writing is a medium of human communication Communication (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area arou ...
in the
Ancient Near East The ancient Near East was the home of early civilization  A civilization (or civilisation) is a complex society A complex society is a concept that is shared by a range of disciplines including anthropology, archaeology, history and s ...
, are the
Bronze Age The Bronze Age is a prehistoric Periodization, period that was characterized by the use of bronze, in some areas proto-writing, and other early features of urban civilization. The Bronze Age is the second principal period of the Three-age sys ...
Egyptian Egyptian describes something of, from, or related to Egypt. Egyptian or Egyptians may refer to: Nations and ethnic groups * Egyptians, a national group in North Africa ** Egyptian culture, a complex and stable culture with thousands of years of r ...
and Sumerian calendars. The Vedic India developed a sophisticated time keeping methodology and calendars for
Vedic upright=1.2, The Vedas are ancient Sanskrit texts of Hinduism. Above: A page from the ''Atharvaveda''. The Vedas (, , ) are a large body of religious texts originating in ancient India. Composed in Vedic Sanskrit, the texts constitute the ol ...

Vedic
rituals. A large number of Ancient Near East calendar systems based on the
Babylonian calendar The Babylonian calendar was a lunisolar calendar with years consisting of 12 lunar months, each beginning when a new lunar phase, crescent moon was first sighted low on the western horizon at sunset, plus an intercalary month inserted as needed b ...
date from the
Iron Age The Iron Age is the final epoch of the three-age division of the prehistory Prehistory, also known as pre-literary history, is the period of human history Human history, or world history, is the narrative of Human, humanity's pa ...
, among them the calendar system of the Persian Empire, which in turn gave rise to the
Zoroastrian calendar Adherents of Zoroastrianism Zoroastrianism or Mazdayasna is an Iranian religion and one of the world's oldest continuously-practiced organized faiths, based on the teachings of the Iranian-speaking prophet Zoroaster Zoroaster (, ; ...
and the
Hebrew calendar The Hebrew calendar ( Hebrew: , ), also called Jewish calendar, is a lunisolar calendar used today for Jewish religious observance, and as an official calendar of the state of Israel Israel (; he, יִשְׂרָאֵל; ar, إِس ...
. A great number of
Hellenic calendars Various ancient Greek calendars began in most states of ancient Greece ''Polis'' (; grc-gre, πόλις ), plural ''poleis'' (, ) literally means "city A city is a large human settlement.Goodall, B. (1987) ''The Penguin Dictionary of Huma ...
developed in
Classical Greece Classical Greece was a period of around 200 years (the 5th and 4th centuries BC) in Ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a civilization belonging to a period of History of Greece, Greek history from the Greek Dar ...
, and in the
Hellenistic period The Hellenistic period spans the period of History of the Mediterranean region, Mediterranean history between the death of Alexander the Great in 323 BC and the emergence of the Roman Empire, as signified by the Battle of Actium in 31  ...
gave rise to both the ancient
Roman calendar The Roman calendar was the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific ...
and to various
Hindu calendars The Hindu calendar or Panchang refers to a set of various lunisolar calendars that are traditionally used in the Indian subcontinent and South-east Asia, with further regional variations for social and Hindu religious purposes. They adopt a sim ...
. According to Yukio Ohashi, Vedanga field developed from actual astronomical studies in ancient Vedic Period and not derived from other cultures. Calendars in antiquity were
lunisolar A lunisolar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day wit ...
, depending on the introduction of
intercalary month Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendar A lunisolar calendar is a calendar in many culture Cul ...
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 Coligny calendar is a Gaulish Gaulish was an ancient Celtic language that was spoken in parts of Continental Europe Mainland or continental Europe is the contiguous continent of Europe, excluding its surrounding islands. It can a ...
. The
Roman calendar The Roman calendar was the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific ...
was reformed by
Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman Roman or Romans most often refers to: *, the capital city of Italy *, Roman civilization from 8th century BC to 5th century AD *, the people of ancient Rome *', shortened ...

Julius Caesar
in 46 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 A month is a unit of time, used with calendars, that is approximately as long as a natural orbital period of the Moon; the words ''month'' and ''Moon'' are cognates. The traditional concept arose with the cycle of Moon phases; such lunar months ("l ...
from the
lunation In lunar calendar A lunar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a sing ...
. The
Islamic calendar The Hijri calendar ( ar, ٱلتَّقْوِيم ٱلْهِجْرِيّ '), also known as the Lunar Hijri calendar and (in English) as the Islamic, Muslim or Arabic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 o ...
is based on the prohibition of intercalation ('' nasi''') by
Muhammad Muhammad ibn AbdullahHe is referred to by many appellations, including Messenger of Allah, The Prophet Muhammad, Allah's Apostle, Last Prophet of Islam, and others; there are also many variant spellings of Muhammad, such as Mohamet, Mohammed, ...

Muhammad
, in Islamic tradition dated to a sermon held on 9
Dhu al-Hijjah Dhu al-Hijjah ( ar, ذُو ٱلْحِجَّة, ', ), also spelled Zu al-Hijjah, is the twelfth and final month in the Islamic calendar. It is a very sacred month in the Islamic calendar, one in which the ''Hajj, Ḥajj'' (Pilgrimage) takes place ...
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 The Gregorian calendar is the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, speci ...
, introduced in 1582 based on the observation of a long-term shift between the Julian calendar and the solar year. There have been several modern proposals for reform of the calendar, such as the
World Calendar #REDIRECT World Calendar#REDIRECT World Calendar The World Calendar is a proposed reform Reform ( lat, reformo) means the improvement or amendment of what is wrong, corrupt, unsatisfactory, etc. The use of the word in this way emerges in the late 1 ...

World Calendar
,
International Fixed Calendar The International Fixed Calendar (also known as the Cotsworth plan, the Cotsworth calendar and the Eastman plan) is a proposed calendar reform designed by Moses B. Cotsworth, first presented in 1902. The solar calendar A solar calendar is a cale ...
,
Holocene calendar The Holocene calendar, also known as the Holocene Era or Human Era (HE), is a year numbering system that adds exactly 10,000 years to the currently dominant ( AD/ BC or CE/BCE Common Era (CE) is one of the year notations used for the Gre ...
, 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.


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
Julian day The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field ou ...
or
Unix Time Current Unix time () Unix time (also known as Epoch time, POSIX time, seconds since the Epoch, or UNIX Epoch time) is a system for describing a point in time Time is the indefinite continued progress of existence and events that occur in an ...
. 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 The Gregorian calendar is the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, speci ...
(and its very similar predecessor, the
Julian calendar The Julian calendar, proposed by Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century B ...
), the
Islamic calendar The Hijri calendar ( ar, ٱلتَّقْوِيم ٱلْهِجْرِيّ '), also known as the Lunar Hijri calendar and (in English) as the Islamic, Muslim or Arabic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 o ...
, the
Solar Hijri calendar The Solar Hijri calendar ( fa, گاه‌شماری هجری خورشیدی, gāhshomāri-ye hejri-ye khorshidi; ps, لمريز لېږدیز کلیز, lamrez legdez kalhandara) is a solar calendar A solar calendar is a calendar A cale ...
and the
Hebrew calendar The Hebrew calendar ( Hebrew: , ), also called Jewish calendar, is a lunisolar calendar used today for Jewish religious observance, and as an official calendar of the state of Israel Israel (; he, יִשְׂרָאֵל; ar, إِس ...
* year, week, and weekday – e.g., the
ISO week date The ISO week date system is effectively a leap week calendar A leap week calendar is a calendar system with a whole number of weeks in a year, and with every year starting on the same weekday. Most leap week calendars are proposed calendar ref ...
Cycles can be synchronized with periodic phenomena: *
Lunar calendar A lunar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day with ...
s are synchronized to the motion of the Moon (
lunar phase The lunar phase or Moon phase is the shape of the Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural s ...
s); an example is the Islamic calendar. *
Solar calendar A solar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day within ...
s are based on perceived
seasonal A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. In ...
changes synchronized to the apparent motion of the Sun; an example is the
Persian calendar The Iranian calendars or Iranian chronology ( fa, گاه‌شماری ایرانی, ''Gāh-Šomāri-ye Irāni'') are a succession of calendars invented or used for over two millennia in Iran Iran ( fa, ایران ), also called Persia ...
. *
Lunisolar calendar A lunisolar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day ...
s are based on a combination of both solar and lunar reckonings; examples include the traditional calendar of China, the
Hindu calendar The Hindu calendar, Panchanga (Sanskrit Sanskrit (, attributively , ''saṃskṛta-'', nominalization, nominally , ''saṃskṛtam'') is a classical language of South Asia belonging to the Indo-Aryan languages, Indo-Aryan branch of the Indo- ...

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.


Solar

Solar calendars assign a ''date'' to each
solar day A synodic day (or synodic rotation period or solar day) is the rotation period, period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time. The synodic day is distinguished from the sidereal ...
. A day may consist of the period between
sunrise Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning. The term can also refer to the entire process of the solar disk crossing the horizon and its accompanying atmospheric optics, atmospheric eff ...

sunrise
and
sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon The horizon is the apparent line that separates earth Earth is the third planet from the Sun and the only astronomical object known to harbor ...

sunset
, with a following period of
night Night (also described as night time, night-time, or nighttime, unconventionally spelled as ''nite'') is the period of :wikt:ambient, ambient darkness from sunset to sunrise during each 24-hour day, when the Sun is below the horizon. The exact ...

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 Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar times are apparent solar time (sundial tim ...
. Other types of calendar may also use a solar day.


Lunar

Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each
lunar phase The lunar phase or Moon phase is the shape of the Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural s ...
cycle. Because the length of the
lunar month In lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's lunar phase, phases (Lunar month#Synodic month, synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly o ...
is not an even fraction of the length of the
tropical year A tropical year (also known as a solar year or tropical period) is the time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past ...
, 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 Tides are the rise and fall of sea level Mean sea level (MSL) (often shortened to sea level) is an average In colloquial, ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of ...

tide
s. An example is the
Islamic calendar The Hijri calendar ( ar, ٱلتَّقْوِيم ٱلْهِجْرِيّ '), also known as the Lunar Hijri calendar and (in English) as the Islamic, Muslim or Arabic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 o ...
. 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.


Lunisolar

A
lunisolar calendar A lunisolar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day ...
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 The Hindu calendar, Panchanga (Sanskrit Sanskrit (, attributively , ''saṃskṛta-'', nominalization, nominally , ''saṃskṛtam'') is a classical language of South Asia belonging to the Indo-Aryan languages, Indo-Aryan branch of the Indo- ...

Hindu calendar
and
Buddhist calendar The Buddhist calendar is a set of lunisolar calendar A lunisolar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calenda ...
that are popular in
South Asia South Asia is the southern region of Asia Asia () is 's largest and most populous , located primarily in the and . It shares the continental of with the continent of and the continental landmass of with both Europe and . Asia cov ...

South Asia
and
Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia or SEA, is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern subregion of Asia, consisting of the regions ...

Southeast Asia
. Another example is the Hebrew calendar which uses a
19-year cycle
19-year cycle
.


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
season A season is a division of the year based on changes in weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or ...

season
s), traditionally used to facilitate the planning of
agricultural Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Exercise trends, Increases in sedentary behaviors su ...

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 A leap year (also known as an intercalary year or wikt:bissextile, bissextile year) is a calendar year that contains an additional day (or, in the case of a lunisolar calendar, a month) added to keep the calendar year synchronized with the astron ...

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. 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 regnal year is a year of the reign of a sovereign, from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, ...
, a calendar based on the reign of their current sovereign. For example, the year 2006 in
Japan Japan ( ja, 日本, or , and formally ) is an island country An island country or an island nation is a country A country is a distinct territory, territorial body or political entity. It is often referred to as the land of an in ...

Japan
is year 18 Heisei, with Heisei being the era name of
Emperor An emperor (from la, imperator The Latin word "imperator" derives from the stem of the verb la, imperare, label=none, meaning 'to order, to command'. It was originally employed as a title roughly equivalent to ''commander'' under the Roma ...
Akihito is a member of the Imperial House of Japan who reigned as the 125th emperor of Japan, according to the List of Emperors of Japan, traditional order of succession, from 7 January 1989 until 30 April 2019, Heisei era. He succeeded to the Chrysan ...
.


Other types


Arithmetical and astronomical

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 The Second Temple (, ), also known in its later years as Herod's Temple, was the reconstructed Jewish holy temple that stood on the Temple Mount The Temple Mount (Hebrew language, Hebrew: , ; "Mount of the House f God, i.e. the Temple in ...

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 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 The Roman calendar was the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific ...
, 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.


Usage

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 Religion is a social Social organisms, including humans, live collectively in interacting populations. This interaction is considered social whether they are aware of it or not, and whether the exchange is voluntary/involuntary. Etymology ...
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 Business is the activity of making one's living or making money by producing or buying and selling products (such as goods and services). Simply put, it is "any activity or enterprise entered into for profit." Having a business name A trad ...

business
and
family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically subject to the same Politic ...

family
calendar to help prevent them from overcommitting their time. Calendars are also used as part of a complete
timekeeping Time is the indefinite continued progress of existence Existence is the ability of an entity to interact with physical or mental reality Reality is the sum or aggregate of all that is real or existent within a system, as opposed to that wh ...
system: date and time of day together specify a moment in
time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

time
. In the modern world, timekeepers can show time, date, and weekday. Some may also show the lunar phase.


Gregorian

The
Gregorian calendar The Gregorian calendar is the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, speci ...
is the ''
de facto ''De facto'' ( ; , "in fact") describes practices that exist in reality, even though they are not officially recognized by laws. It is commonly used to refer to what happens in practice, in contrast with ''de jure'' ("by law"), which refers to th ...
'' 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 A tropical year (also known as a solar year or tropical period) is the time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past ...
. There is a lunar aspect which approximates the position of the moon during the year, and is used in the calculation of the date of Easter. 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 The Julian calendar, proposed by Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century B ...
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 Roman or Romans most often refers to: *Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Laz ...

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 inherited from the medieval convention established by
Dionysius Exiguus Dionysius Exiguus (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of t ...

Dionysius Exiguus
and associated with the Julian calendar. The year number is variously given as AD (for ''
Anno Domini The terms (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendar The Gregorian calendar is the used in most of the world. It was introduced in October 1582 by as a modification of the , r ...
'') or CE (for ''
Common Era Common Era (CE) is one of the year notations used for the Gregorian calendar (and its predecessor, the Julian calendar), the world's most widely used calendar era. Before the Common Era (BCE) is the era before CE. BCE and CE are alternatives ...
'' or ''Christian Era'').


Religious

The most important use of pre-modern calendars is keeping track of the
liturgical year The liturgical year, also known as the church year or Christian year, as well as the kalendar, consists of the cycle of liturgy, liturgical seasons in Christian churches that determines when feast days, including Calendar of saints, celebrati ...

liturgical year
and the observation of religious feast days. While the Gregorian calendar is itself historically motivated to the calculation of the
Easter date As a moveable feast, the date of Easter is determined in each year through a calculation known as ''computus'' (Latin for 'computation'). Easter is celebrated on the first Sunday after the Paschal full moon, which is the first full moon on or af ...
, 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. Eastern Christians, including the
Orthodox Church Orthodox Church may refer to: * Eastern Orthodox Church The Eastern Orthodox Church, also called the Orthodox Church, is the List of Christian denominations by number of members, second-largest Christian church, with approximately 220 mi ...
, use the
Julian calendar The Julian calendar, proposed by Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century B ...
. The
Islamic calendar The Hijri calendar ( ar, ٱلتَّقْوِيم ٱلْهِجْرِيّ '), also known as the Lunar Hijri calendar and (in English) as the Islamic, Muslim or Arabic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 o ...
or Hijri calendar is a
lunar calendar A lunar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day with ...
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 AD 622) With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately every 33 Islamic years. Various
Hindu calendar The Hindu calendar, Panchanga (Sanskrit Sanskrit (, attributively , ''saṃskṛta-'', nominalization, nominally , ''saṃskṛtam'') is a classical language of South Asia belonging to the Indo-Aryan languages, Indo-Aryan branch of the Indo- ...

Hindu calendar
s remain in use in the Indian subcontinent, including the Nepali calendars,
Bengali calendar The Bengali Calendar or Bangla Calendar ( bn, বঙ্গাব্দ , , Baṅgābda) is a luni-solar calendar A lunisolar calendar is a calendar in many culture Culture () is an umbrella term which encompasses the social behavior and Nor ...
,
Malayalam calendar Malayalam Calendar or Kollam Era is a solar Solar may refer to: Astronomy * Of or relating to the Sun. ** A solar telescope 175px, The Swedish 1-m Solar Telescope at Roque de los Muchachos Observatory, La Palma in the Canary Islands. A sol ...
,
Tamil calendar The Tamil calendar is a Sidereal time, sidereal Hindu calendar used by the Tamil people of the Indian subcontinent. It is also used in Puducherry (union territory), Puducherry, and by the Tamil people, Tamil population in Sri Lanka, Malaysia, Sing ...
,
Vikrama Samvat Vikram Samvat (Hindi Hindi ( Devanagari: हिन्दी, IAST/ ISO 15919: ''Hindī''), or more precisely Modern Standard Hindi ( Devanagari: मानक हिन्दी, IAST/ ISO 15919: ''Mānak Hindī''), is an Indo-Aryan lan ...
used in Northern India, and
Shalivahana Shalivahana (International Alphabet of Sanskrit Transliteration, IAST: Śālivāhana) was a legendary emperor of ancient India, who is said to have ruled from Pratishthana (present-day Paithan, Maharashtra). He is believed to be based on a Satavaha ...
calendar in the Deccan states. The
Buddhist calendar The Buddhist calendar is a set of lunisolar calendar A lunisolar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calenda ...
and the traditional lunisolar calendars of
Cambodia Cambodia (; also Kampuchea ; km, កម្ពុជា, ), officially the Kingdom of Cambodia, is a country located in the southern portion of the Indochinese peninsula in Southeast Asia. It is in area, bordered by Thailand to Cambodia–T ...

Cambodia
,
Laos , national_anthem = "Pheng Xat Lao "Pheng Sat Lāo" () is the national anthem A national anthem is a song that officially symbolizes a country A country is a distinct territory, territorial body or political entity. It is often r ...

Laos
,
Myanmar Myanmar, ); UK pronunciations: US pronunciations incl. . Note: Wikipedia's IPA conventions require indicating /r/ even in British English although only some British English speakers pronounce r at the end of syllables. As John C. Wells, John ...

Myanmar
,
Sri Lanka Sri Lanka (, ; si, ශ්‍රී ලංකාව, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon, and officially the Democratic Socialist Republic of Sri Lanka, is ...

Sri Lanka
and
Thailand Thailand ( th, ประเทศไทย), historically known as Siam, () officially the Kingdom of Thailand, is a country in Southeast Asia. It is located at the centre of the Mainland Southeast Asia, Indochinese Peninsula, spanning , wi ...

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 Vedanga Jyotisha (), or Jyotishavedanga (), is one of earliest known Indian texts on astrology Astrology is a pseudoscience that claims to divination, divine information about human affairs and terrestrial events by studying the mov ...
of Lagadha, standardized in the '' Sūrya Siddhānta'' and subsequently reformed by astronomers such as Āryabhaṭa (AD 499),
Varāhamihira Varāhamihira ( 505 – 587), also called Varāha or Mihira, was a Hindu astrologer, astronomer, and polymath who lived in Ujjain (Madhya Pradesh, India). He was born in the Avanti region, roughly corresponding to modern-day Malwa (part of ...
(6th century) and
Bhāskara II Bhāskara (c. 1114–1185) also known as Bhāskarācārya ("Bhāskara, the teacher"), and as Bhāskara II to avoid confusion with Bhāskara I, was an Indian people, Indian Indian mathematicians, mathematician and astronomer. He was born in Bij ...
(12th century). The
Hebrew calendar The Hebrew calendar ( Hebrew: , ), also called Jewish calendar, is a lunisolar calendar used today for Jewish religious observance, and as an official calendar of the state of Israel Israel (; he, יִשְׂרָאֵל; ar, إِس ...
is used by
Jews Jews ( he, יְהוּדִים ISO 259-2 ISO The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm (social), ...

Jews
worldwide for religious and cultural affairs, also influences civil matters in Israel (such as
national holidaysNational holiday may refer to: *National day, a day when a nation celebrates a very important event in its history, such as its establishment *Public holiday, a holiday established by law, usually a day off for at least a portion of the workforce, s ...
) and can be used business dealings (such as for the dating of
cheque A cheque, or check (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the Unit ...
s). Followers of the
Baháʼí Faith The Baháʼí Faith (; fa , بهائی ') is a new religion teaching the essential worth of all religions and the unity of all people. Established by Baháʼu'lláh Baháʼu'lláh (12 November 1817 – 29 May 1892) was a Persian ...
use the
Baháʼí calendar The Badíʻ calendar used in the Baháʼí Faith The Baháʼí Faith (; fa , بهائی ') is a new religion teaching the essential worth of all religions and the unity of all people. Established by Baháʼu'lláh Baháʼu'lláh (1 ...
. 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 Chinese can refer to: * Something related to China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the List of countries and dependencies by population, world's most populous country, with a populat ...
,
Hebrew Hebrew (, , or ) is a Northwest Semitic languages, Northwest Semitic language of the Afroasiatic languages, Afroasiatic language family. Historically, it is regarded as one of the spoken languages of the Israelites and their longest-survivi ...
,
Hindu Hindus (; ) are persons who regard themselves as culturally, ethnically, or religiously adhering to aspects of Hinduism Hinduism () is an Indian religion Indian religions, sometimes also termed Dharmic religions or Indic re ...

Hindu
, and Julian calendars are widely used for religious and social purposes. The Iranian (Persian) calendar is used in
Iran Iran ( fa, ایران ), also called Persia, and officially the Islamic Republic of Iran, is a country in Western Asia Western Asia, West Asia, or Southwest Asia, is the westernmost subregion A subregion is a part of a larger regio ...

Iran
and some parts of
Afghanistan Afghanistan (; Pashto Pashto (,; / , ), sometimes spelled Pukhto or Pakhto, is an Eastern Iranian language The Eastern Iranian languages are a subgroup of the Iranian languages The Iranian or Iranic languages are a branch of t ...

Afghanistan
. The
Assyrian calendar The Assyrian calendar ( aii, ܣܘܼܪܓܵܕ݂ܵܐ ܐܵܬ݂ܘܼܪܵܝܵܐ ) is a solar calendar A solar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, w ...
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 The Ethiopian calendar (Ge'ez: ዓዉደ ወርህ; Tigrinya Tigrinya (ትግርኛ; also spelled Tigrigna) is a Semitic language spoken in Eritrea Eritrea ( ), officially the State of Eritrea, is a country in Eastern Africa, wi ...
or Ethiopic calendar is the principal calendar used in
Ethiopia Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country A landlocked country is a country A country is a distinct territory, territorial body or political entity. It is often referred to as the ...

Ethiopia
and
Eritrea Eritrea ( ), officially the State of Eritrea, is a country in the Horn of Africa The Horn of Africa (HoA), also known as the Somali Peninsula, is a large peninsula A peninsula ( la, paeninsula from ' "almost" and ' "island") is a ...

Eritrea
, with the Oromo calendar also in use in some areas. In neighboring
Somalia Somalia,, Osmanya script: 𐒈𐒝𐒑𐒛𐒐𐒘𐒕𐒖; ar, الصومال, aṣ-Ṣūmāl officially the Federal Republic of SomaliaThe ''Federal Republic of Somalia'' is the country's name per Article 1 of thProvisional Constitutio ...

Somalia
, the
Somali calendar The Somali calendar ( so, Soomaali tiro ammin) is based on both the solar and lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's phases ( synodic months), in contrast to solar calendars, whose annual cyc ...
co-exists alongside the Gregorian and Islamic calendars. In
Thailand Thailand ( th, ประเทศไทย), historically known as Siam, () officially the Kingdom of Thailand, is a country in Southeast Asia. It is located at the centre of the Mainland Southeast Asia, Indochinese Peninsula, spanning , wi ...

Thailand
, where the
Thai solar calendar The Thai solar calendar ( th, ปฏิทินสุริยคติ, , "solar calendar") was adopted by King Chulalongkorn Chulalongkorn ( th, จุฬาลงกรณ์, 20 September 1853 – 23 October 1910) was the fifth monarc ...
is used, the months and days have adopted the western standard, although the years are still based on the traditional
Buddhist calendar The Buddhist calendar is a set of lunisolar calendar A lunisolar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calenda ...
.


Fiscal

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 Diwali (; Deepavali (IAST: ''dīpāvalī'') or Divali; related to Diwali (Jainism), Jain Diwali, Bandi Chhor Divas, Tihar (festival), Tihar, Swanti (festival), Swanti, Sohrai and Bandna) is a festival of lights and one of the major festival ...

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.


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 appointment book in the form of a pocket calendar (or
personal organizer A personal organizer, datebook, date log, daybook, day planner, personal analog assistant, personal planner, year planner, or agenda (from Latin ''agenda'' – things to do), is a small book or binder that is designed to be portable. It usually c ...
), desktop calendar, a
wall calendar A calendar is used to display dates and related information, usually in a table (information), table format. Calendars are used to plan future events and keep track of appointments, and so a typical calendar will include days of the week, week numbe ...
, 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. 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 in
common year A common year is a calendar year A calendar is a system of organizing days. This is done by giving names to periods of time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in ...
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

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 PDA may refer to: Science and technology * Patron-driven acquisitionPatron-driven acquisition (PDA), also referred to as demand-driven acquisition (DDA), is a model of library collection development in which a library only purchases materials whe ...
, EDAs, and
smartphones A smartphone is a portable device A mobile device (or handheld computer) is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can per ...

smartphones
. The software may be a local package designed for individual use (e.g.,
Lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan ...
extension for
Mozilla Thunderbird Mozilla Thunderbird is a free and open-source Free and open-source software (FOSS) is software that is both free software and open-source software where anyone is free software license, freely licensed to use, copy, study, and change the sof ...
,
Microsoft Outlook Microsoft Outlook is a personal information manager software system from Microsoft, available as a part of the Microsoft Office suite. Though primarily an email client, Outlook also includes such functions as Calendaring software, calendaring ...
without Exchange Server, or
Windows Calendar Calendar is a personal calendaring software, calendar application made by Microsoft for Microsoft Windows. It offers synchronization of calendars using Microsoft Exchange Server, Outlook.com Apple's iCloud calendar service, and Google Calendar. It ...
) or maybe a networked package that allows for the sharing of information between users (e.g.,
Mozilla Sunbird Mozilla Sunbird is a discontinued free and open-source Free and open-source software (FOSS) is software that is both free software and open-source software where anyone is free software license, freely licensed to use, copy, study, and change ...

Mozilla Sunbird
,
Windows Live Calendar Outlook.com is a personal information manager web app from Microsoft consisting of webmail, Calendaring software, calendaring, Address book, contacts, and Task management, tasks services. Founded in 1996 by Sabeer Bhatia and Jack Smith (Hotmail ...
,
Google Calendar Google Calendar is a time-management and scheduling calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of ...

Google Calendar
, or
Microsoft Outlook Microsoft Outlook is a personal information manager software system from Microsoft, available as a part of the Microsoft Office suite. Though primarily an email client, Outlook also includes such functions as Calendaring software, calendaring ...
with Exchange Server).


See also

*
List of calendars This is a list of calendars. Included are historical calendars as well as proposed ones. Historical calendars are often grouped into larger categories by cultural sphere or historical period; thus O'Neil (1976) distinguishes the groupings Egypti ...
*
Advent calendar An Advent calendar is a special calendar used to count the days of Advent in anticipation of Christmas. Since the date of the Advent Sunday, First Sunday of Advent varies, falling between November 27 and December 3 inclusive, many Advent calendar ...
*
Calendar reform Calendar reform or calendrical reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design. Principles The prime objective of a calendar is to unambiguou ...
*
Calendrical calculationA calendrical calculation is a calculation A calculation is a deliberate process that transforms one or more inputs into one or more results. The term is used in a variety of senses, from the very definite arithmetic Arithmetic (from the Ancien ...
*
Docket (court) A docket in the United States is the official summary of proceedings in a court of law. In the United Kingdom in modern times it is an official document relating to delivery of something, with similar meanings to these two elsewhere. In the late ...
*
History of calendars The history of calendars, that is, of people creating and using methods for keeping track of days and larger divisions of time, covers a practice with ancient roots. Archeologists have reconstructed methods of timekeeping that go back to prehis ...
*
Horology Horology ("the study of time", related to Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meani ...
* List of international common standards * List of unofficial observances by date *
Real-Time Clock A real-time clock (RTC) is an electronic device (most often in the form of an integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuit ...
(RTC), which underlies the Calendar software on modern computers. *
Unit of time A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The SI base unit, base unit of time in the International System of Units (SI) and by extension most of the Western world, is the second, ...


References


Citations


Sources

* * * * * * * * * * *


Further reading

* * * *


External links

* *
Calendar converter, including all major civil, religious and technical calendars.
{{Authority control Units of time