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) that illustrates the system
(for example, a desktop calendar) 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
Calendars can be classified by what their
cycles are synchronised with:
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
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.
An arbitrary calendar is not synchronized to any external
phenomenon; for example the week cycle.
A calendar can also be acyclic. For example, the Julian day
used by astronomers is a simple linear count of days.
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.
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 and International Fixed 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. Holocene calendar is another one for
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.
Other calendar types
Complete and incomplete calendars
Pragmatic, theoretical and
Calendars may be either complete or incomplete. Complete calendars
provide a way of naming each consecutive day, while incomplete
calendars do not. The early Roman calendar, which had no way
of designating the days of the winter months other than to
lump them together as "winter", is an example of
an incomplete calendar, while the Gregorian calendar is an
example of a complete calendar.
Calendars 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 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 China
and India (along with the Indian national calendar). Due to
the Gregorian calendar's obvious connotations with Christianity
and Jesus, non-Christians sometimes justify its usage by replacing
the traditional era notations "AD" and "BC"
with "CE" and "BCE" (Common Era). The Hindu
calendars are some of the most ancient calendars of the world.
Gregorian calendar is much more widely used in Israel's business
and day-to-day affairs.
Also, the Persian calendar is used in Iran
and Afghanistan. The Islamic calendar is used by Muslims the
world over. The Chinese, Hebrew, Hindu, and Julian calendars
are widely used for religious and/or social purposes.
Even where there is a commonly used calendar
such as the Gregorian calendar, alternate calendars may also
be used, such as a fiscal calendar.