Calendar year

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A calendar year begins on the New Year's Day of the given calendar system and ends on the day before the following New Year's Day, and thus consists of a whole number of days.

Contents

The Gregorian calendar year, which is in use as civil calendar in most of the world, begins on January 1 and ends on December 31. [1] It has a length of 365 days in an ordinary year but, in order to reconcile the calendar year with the astronomical cycle, it has 366 days in a leap year. With 97 leap years every 400 years, the Gregorian calendar year has an average length of 365.2425 days.

Other formula-based calendars can have lengths which are further out of step with the solar cycle: for example, the Julian calendar has an average length of 365.25 days, and the Hebrew calendar has an average length of 365.2468 days. The Lunar Hijri calendar ("Islamic calendar") is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. [a] The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the calendar year in most calendars.

A year can also be measured by starting on any other named day of the calendar, and ending on the day before this named day in the following year. [2] This may be termed a "year's time", but is not a "calendar year".

Quarter year

The calendar year can be divided into four quarters, [3] often abbreviated as Q1, Q2, Q3, and Q4. Since they are three months each, they are also called trimesters. In the Gregorian calendar:

In some domains, weeks are preferred over months for scheduling and reporting, so they use quarters of exactly 13 weeks each, often following ISO week date conventions. One in five to six years has a 53rd week which is usually appended to the last quarter. It is then 98 days instead of 91 days long, which complicates comparisons.

In the Chinese calendar, the quarters are traditionally associated with the 4 seasons of the year:

Quadrimester

The calendar year can also be divided into quadrimesters (from French quadrimestre), [5] lasting for four months each. They can also be called the early, middle, or late parts of the year. In the Gregorian calendar:

Semester

The calendar year can also be divided into semesters, [6] lasting six months each and often being abbreviated as S1 and S2. In the Gregorian calendar:

See also

Today
Sunday
Gregorian calendar January 12, 2025
Islamic calendar 12 Rajab, 1446 AH
Hebrew calendar 12 Tevet, AM 5785
Coptic calendar Tobi 4, 1741 AM
Solar Hijri calendar 23 Dey, 1403 SH
Bengali calendar Poush 28, 1431 BS
Julian calendar 30 December 2024
Byzantine calendar 12 January 7533

Notes

  1. The other Islamic calendar, observed in Iran, is the Solar Hijri calendar. It runs from spring equinox to spring equinox.

Related Research Articles

<span class="mw-page-title-main">Calendar</span> System for organizing the days of year

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 date is the designation of a single and specific day within such a system. A calendar is also a physical record of such a system. A calendar can also mean a list of planned events, such as a court calendar, or a partly or fully chronological list of documents, such as a calendar of wills.

<span class="mw-page-title-main">Islamic calendar</span> Lunar calendar used by Muslims

The Hijri calendar, or Arabic calendar, also known in English as the Muslim calendar and Islamic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to determine the proper days of Islamic holidays and rituals, such as the annual fasting and the annual season for the great pilgrimage. In almost all countries where the predominant religion is Islam, the civil calendar is the Gregorian calendar, with Syriac month-names used in the Levant and Mesopotamia, but the religious calendar is the Hijri one.

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 calendars may require intercalations of days or months.

The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year. The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Amazigh people.

A leap year is a calendar year that contains an additional day compared to a common year. The 366th day is added to keep the calendar year synchronised with the astronomical year or seasonal year. Since astronomical events and seasons do not repeat in a whole number of days, calendars having a constant number of days each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("intercalating") an additional day—a leap day—or month—a leap month—into some years, the drift between a civilization's dating system and the physical properties of the Solar System can be corrected.

A month is a unit of time, used with calendars, that is approximately as long as a natural phase cycle of the Moon; the words month and Moon are cognates. The traditional concept of months arose with the cycle of Moon phases; such lunar months ("lunations") are synodic months and last approximately 29.53 days, making for roughly 12.37 such months in one Earth year. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months, based on the Moon's orbital period with respect to the Earth–Sun line, are still the basis of many calendars today and are used to divide the year.

The Revised Julian calendar, or less formally the new calendar and also known as the Milanković calendar, is a calendar proposed in 1923 by the Serbian scientist Milutin Milanković as a more accurate alternative to both Julian and Gregorian calendars. At the time, the Julian calendar was still in use by all of the Eastern Orthodox Church and affiliated nations, while the Catholic and Protestant nations were using the Gregorian calendar. Thus, Milanković's aim was to discontinue the divergence between the naming of dates in Eastern and Western churches and nations. It was intended to replace the Julian calendar in Eastern Orthodox Churches and nations. From 1 March 1600 through 28 February 2800, the Revised Julian calendar aligns its dates with the Gregorian calendar, which had been proclaimed in 1582 by Pope Gregory XIII.

<span class="mw-page-title-main">Year</span> Time of one planets orbit around a star

The year is a unit of time based on the roughly 365¼ days taken by the Earth to revolve around the Sun. The contemporary calendar year, based on the Gregorian solar calendar, approximates this cycle.

A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar. The main other types of calendar are lunar calendar and lunisolar calendar, whose months correspond to cycles of Moon phases. The months of the Gregorian calendar do not correspond to cycles of the Moon phase.

<span class="mw-page-title-main">Date of Easter</span>

As a moveable feast, the date of Easter is determined in each year through a calculation known as computus paschalis – often simply Computus – or as paschalion particularly in the Orthodox church. Easter is celebrated on the first Sunday after the Paschal full moon. Determining this date in advance requires a correlation between the lunar months and the solar year, while also accounting for the month, date, and weekday of the Julian or Gregorian calendar. The complexity of the algorithm arises because of the desire to associate the date of Easter with the date of the Jewish feast of Passover which, Christians believe, is when Jesus was crucified.

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.

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 reforms to the civil calendar, in order to achieve a perennial calendar. Some, however, such as the ISO week date calendar, are simply conveniences for specific purposes.

<span class="mw-page-title-main">Birkat Hachama</span> Jewish blessing, thanking God for creating the sun. Recited once every 28 years.

Birkat Hachama refers to a rare Jewish blessing that is recited to the Creator, thanking God for creating the sun. The blessing is recited when the Sun completes its cycle every 28 years on a Tuesday at sundown. Jewish tradition says that when the Sun completes this cycle, it has returned to its position when the world was created. Because the blessing needs to be said when the Sun is visible, the blessing is postponed to the following day, on Wednesday morning.

The Hijri year or era is the era used in the Islamic lunar calendar. It begins its count from the Islamic New Year in which Muhammad and his followers migrated from Mecca to Yathrib in 622 CE. This event, known as the Hijrah, is commemorated in Islam for its role in the founding of the first Muslim community (ummah). Currently, the Hijri year is 1446 AH.

<span class="mw-page-title-main">Unit of time</span> Measurement unit for time

A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1."

The ISO week date system is effectively a leap week calendar system that is part of the ISO 8601 date and time standard issued by the International Organization for Standardization (ISO) since 1988 and, before that, it was defined in ISO (R) 2015 since 1971. It is used (mainly) in government and business for fiscal years, as well as in timekeeping. This was previously known as "Industrial date coding". The system specifies a week year atop the Gregorian calendar by defining a notation for ordinal weeks of the year.

The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day "tropical" or "solar" year that is determined by the Earth's revolution around the Sun.

The Hanke–Henry Permanent Calendar (HHPC) is a proposal for calendar reform. It is one of many examples of leap week calendars, calendars that maintain synchronization with the solar year by intercalating entire weeks rather than single days. It is a modification of a previous proposal, Common-Civil-Calendar-and-Time (CCC&T). With the Hanke–Henry Permanent Calendar, every calendar date always falls on the same day of the week. A major feature of the calendar system is the abolition of time zones.

<span class="mw-page-title-main">Solar Hijri calendar</span> Official calendar of Iran

The Solar Hijri calendar is the official calendar of Iran. It is a solar calendar and is the one Iranian calendar that is the most similar to the Gregorian calendar, being based on the Earth's orbit around the Sun. It begins on the March equinox as determined by the astronomical calculation for the Iran Standard Time meridian and has years of 365 or 366 days. It is sometimes also called the Shamsi calendar, Khorshidi calendar, or Persian calendar. It is abbreviated as SH, HS, AP, or, sometimes as AHSh, while the lunar Hijri calendar is usually abbreviated as AH.

References

  1. "calendar year". Cambridge Dictionary . Archived from the original on 2022-05-03. Retrieved 2025-01-12.
  2. "calendar year". Merriam-Webster. Retrieved 6 August 2014.
  3. "Calendar quarter". Merriam-Webster (Merriam-Webster.com). Retrieved 23 December 2024.
  4. "Calendar quarter". Cambridge Business English Dictionary (Dictionary.Cambridge.org). Retrieved 23 December 2024.
  5. "QUADRIMESTRE : Définition de QUADRIMESTRE". Centre National de Ressources Textuelles et Lexicales (cnrtl.fr). Retrieved 4 January 2025.
  6. "Semester". Merriam-Webster (Merriam-Webster.com). Retrieved 4 January 2025.