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 (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 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, 11, 12, or 13 dates were skipped
when the Julian calendar was replaced by the Gregorian calendar by
various countries.
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.
Main article: Calendar reform
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.
Main article: Lunar calendar
Not all calendars use the solar year as a unit. A lunar calendar is one in
which days are numbered within each lunar phase cycle. Because the
length of the lunar month is not an even fraction of the length of the
tropical year, a purely lunar calendar quickly drifts against the seasons. 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.
Fiscal calendars
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 4 January Gregorian.
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 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.
Uses
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.
Kurdish calendar is a Solar calendar used among the Kurdish people. The
Kurdish year begins on March 21st, at the time of vernal equinox.Kurdish
calendar began 612 BC, when the Medes conquered Nineveh and Assyria,
which marks and symbolizes the end of lowland tyranny. The year 2007
corresponds to the Kurdish year 2619.
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.
Physical calendars
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.
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