Several non-standard dates are used in calendars for various purposes: some are expressly fictional, some are intended to produce a rhetorical effect (such as sarcasm), and others attempt to address a particular mathematical, scientific or accounting requirement or discrepancy within the calendar system.
January 0 or 0 January is an alternative name for December 31. January 0 is the day before January 1 in an annual ephemeris. It keeps the date in the year for which the ephemeris was published, thus avoiding any reference to the previous year, even though it is the same day as December 31 of the previous year. January 0 also occurs in the epoch for the ephemeris second, "1900 January 0 at 12 hours ephemeris time". [1] 1900 January 0 (at Greenwich Mean Noon) was also the epoch used by Newcomb's Tables of the Sun, which became the epoch for the Dublin Julian day. [2]
February 30 or 30 February is a date that does not occur on the Gregorian calendar, where the month of February contains only 28 days, or 29 days in a leap year. However, from a historical perspective February 30 has been used at least once and appears in some reform calendars.
The thirteenth-century scholar Johannes de Sacrobosco claimed that in the Julian calendar, February had 30 days in leap years from 45 BC until 8 BC, when Augustus allegedly shortened February by one day to give that day to the month of August named after him so that it had the same length as the month of July named after his adoptive father, Julius Caesar. However, all historical evidence refutes Sacrobosco, including dual dates with the Alexandrian calendar. [3]
February 30 was a day that happened in Sweden, 1712. [4] This occurred because, instead of changing from the Julian calendar to the Gregorian calendar by omitting a block of consecutive days, as had been done in other countries, the Swedish Empire planned to change gradually by omitting all leap days from 1700 to 1740, inclusive. Although the leap day was omitted in February 1700, the Great Northern War began later that year, diverting the attention of the Swedes from their calendar so that they did not omit leap days on the next two occasions; 1704 and 1708 remained leap years. [5]
To avoid confusion and further mistakes, the Julian calendar was restored in 1712 by adding an extra leap day, thus giving that year the only known actual use of February 30 in a calendar. That day corresponded to February 29 in the Julian calendar and to March 11 in the Gregorian calendar. [5] [6] The Swedish conversion to the Gregorian calendar was finally accomplished in 1753, when February 17 was followed by March 1. [5]
Artificial calendars may also have 30 days in February. For example, in a climate model the statistics may be simplified by having 12 months of 30 days. The Hadley Centre General Circulation Model is an example. [7]
February 31 or 31 February is exceptionally used on gravestones when the date is unknown, [8] [ better source needed ] or, in at least one case, out of supposed superstition (more likely an error). [9] [10]
May 35 or 35 May is used in mainland China to avoid censorship when referring to the Tiananmen Square protests of 1989, where the official names are strictly censored by the national government, and the event is normally referred to as June 4. [11] It is also used in the title of The 35th of May, or Conrad's Ride to the South Seas , a German children's novel published in 1931.
December 31.5 GMT in 1924 almanacs was an instant defined to resolve the contrast between two different conventions in defining the civil time of referring to midnight as zero hours. [12]
The LearAvia Lear Fan aircraft test flight had British government "funding that expired at the end of that year." After the cancellation of a planned test flight on December 31, 1980, due to technical issues, the first prototype made its maiden flight on January 1, 1981, but a sympathetic government official recorded the date as "December 32, 1980". [13]
Microsoft Excel displays the day before January 1, 1900 (the earliest date it can represent) as January 0, 1900. [14] It also treats 1900 incorrectly as a leap year (whereas only centuries divisible by 400 are), so it displays the day before March 1, 1900 as the non-existent February 29 instead of February 28. This means March 1, 1900 is the earliest date that can be used reliably in Excel.
February 31 is used (along with February 32 and February 33) for calculating weather data. [15] [16]
Other non-standard dates are sometimes used in software engineering. For example, Java (specifically the java.util.Calendar class) allows dates such as February 0 (= January 31) and April 31 (= March 1).
March 0 is used in astronomy. [17] [ clarification needed ]
In the works of J. R. R. Tolkien, the Hobbits have developed the Shire Reckoning. According to Appendix D of The Lord of the Rings , this calendar has arranged the year in 12 months of 30 days each. The month the Hobbits call Solmath is rendered in the text as February, and therefore the date February 30 exists in the narrative. [18]
February 30, 1951, is the last night of the world in Ray Bradbury's short story "Last Night of the World". [19]
June 31 is a fictional date in the Soviet film 31 June . It is also the date of a fictional RAF raid on Germany in Len Deighton's 1970 novel Bomber .
December 32 or 32 December is the date of Hogswatchnight in Hogfather by Terry Pratchett. It has also been used as a title for various works.
The children's book Please Try to Remember the First of Octember! by Dr. Seuss narrates many delightful things which are supposed to happen starting on the first day of the fictional month of Octember.
March 0 is used in Doomsday algorithm calculations. [20] March 2 was celebrated as February 30 by Lin-Manuel Miranda and Weird Al Yankovic for the release date of Yankovic's "The Hamilton Polka". [21]
In November 2010 it was discovered that a Hanshin Tigers wall calendar incorrectly included the date November 31. Fans who had bought the calendar were given a sticker to cover up the date, and reprinted calendars were sent. [22]
In the episode "94 Meetings" of the sitcom Parks and Recreation, Ron Swanson is forced to deal with 94 meetings in a single day because his assistant, April, scheduled them all for March 31st (instead of the common fake date of February 31st), mistakenly believing it was not a real date.
Soul band Black Pumas included a song named "OCT 33" on their Grammy-nominated 2019 eponymous album.
The 1998 French-language film August 32nd on Earth was written and directed by Denis Villeneuve, and was his feature film directoral debut.
In Bangladeshi popular culture, August 5, 2024 was celebrated as July 36, when Prime minister Sheikh Hasina resigned and fled the country amid the mass uprising, in remembrance of those died in the students' job quota reform movement on the month of July 2024.
Because evening out the lengths of the months is part of the rationale for reforming the calendar, some reform calendars, such as the World Calendar and the Hanke–Henry Permanent Calendar, contain a 30-day February. The Symmetry454 calendar assigns 35 days to February, May, August, and November, as well as December in a leap year.
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.
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.
A year is the time taken for astronomical objects to complete one orbit. For example, a year on Earth is the time taken for Earth to revolve around the Sun. Generally, a year is taken to mean a calendar year, but the word is also used for periods loosely associated with the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. The term can also be used in reference to any long period or cycle, such as the Great Year.
The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events.
The Coptic calendar, also called the Alexandrian calendar, is a liturgical calendar used by the farming populace in Egypt and also used by the Coptic Catholic Church. It was used for fiscal purposes in Egypt until the adoption of the Gregorian calendar on 11 September 1875. This calendar is based on the ancient Egyptian calendar. To avoid the calendar creep of the latter, a reform of the ancient Egyptian calendar was introduced at the time of Ptolemy III which consisted of adding an extra day every fourth year. However, this reform was opposed by the Egyptian priests, and the reform was not adopted until 25 BC, when the Roman Emperor Augustus imposed the Decree upon Egypt as its official calendar. To distinguish it from the Ancient Egyptian calendar, which remained in use by some astronomers until medieval times, this reformed calendar is known as the Coptic or Alexandrian calendar. Its years and months coincide with those of the Ethiopian calendar but have different numbers and names.
As a moveable feast, the date of Easter is determined in each year through a calculation known as computus. 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.
The Swedish calendar or Swedish style was a calendar in use in Sweden and its possessions from 1 March 1700 until 30 February 1712. It was one day ahead of the Julian calendar and ten days behind the Gregorian calendar. Easter was calculated astronomically, with a minor exception, from 1740 to 1844.
Dominical letters or Sunday letters are a method used to determine the day of the week for particular dates. When using this method, each year is assigned a letter depending on which day of the week the year starts. The Dominical letter for the current year 2024 is GF.
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.
The determination of the day of the week for any date may be performed with a variety of algorithms. In addition, perpetual calendars require no calculation by the user, and are essentially lookup tables. A typical application is to calculate the day of the week on which someone was born or a specific event occurred.
In astronomy, a Julian year is a unit of measurement of time defined as exactly 365.25 days of 86400 SI seconds each. The length of the Julian year is the average length of the year in the Julian calendar that was used in Western societies until the adoption of the Gregorian Calendar, and from which the unit is named. Nevertheless, because astronomical Julian years are measuring duration rather than designating dates, this Julian year does not correspond to years in the Julian calendar or any other calendar. Nor does it correspond to the many other ways of defining a year.
Old Style (O.S.) and New Style (N.S.) indicate dating systems before and after a calendar change, respectively. Usually, they refer to the change from the Julian calendar to the Gregorian calendar as enacted in various European countries between 1582 and 1923.
The Ethiopian calendar, or Ge'ez calendar is the official state civil calendar of Ethiopia and serves as an unofficial customary cultural calendar in Eritrea, and among Ethiopians and Eritreans in the diaspora. It is also an ecclesiastical calendar for Ethiopian Christians and Eritrean Christians belonging to the Orthodox Tewahedo Churches, Eastern Catholic Churches, and Eastern Protestant Christian P'ent'ay Churches. The Ethiopian calendar is a solar calendar that has much in common with the Coptic calendar of the Coptic Orthodox Church of Alexandria and Coptic Catholic Church, but like the Julian calendar, it adds a leap day every four years without exception, and begins the year on 11 or 12th of September in the Gregorian calendar. A gap of seven to eight years between the Ethiopian and Gregorian calendars results from an alternative calculation in determining the date of the Annunciation.
The Indian national calendar, called the Shaka calendar or Śaka calendar, is a solar calendar that is used alongside the Gregorian calendar by The Gazette of India, in news broadcasts by All India Radio, and in calendars and official communications issued by the Government of India. Śaka Samvat is generally 78 years behind the Gregorian calendar, except from January–March, when it is behind by 79 years.
An ordinal date is a calendar date typically consisting of a year and an ordinal number, ranging between 1 and 366, representing the multiples of a day, called day of the year or ordinal day number. The two parts of the date can be formatted as "YYYY-DDD" to comply with the ISO 8601 ordinal date format. The year may sometimes be omitted, if it is implied by the context; the day may be generalized from integers to include a decimal part representing a fraction of a day.
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 proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to the dates preceding its official introduction in 1582. In nations that adopted the Gregorian calendar after its official and first introduction, dates occurring in the interim period of 15 October 1582 to the date on which the pertinent nation adopted the Gregorian calendar and abandoned the Julian calendar are sometimes 'Gregorianized' also. For example, the birthday of U.S. President George Washington was originally dated 11 February 1731 because Great Britain, of which he was born a subject, used the Julian calendar and dated the beginning of English years as 25 March. After Great Britain switched to the Gregorian calendar, Washington's birthday was dated 22 February 1732 proleptically, according to the Gregorian calendar applied backward. This remains the modern dating of his birthday.
The tables below list equivalent dates in the Julian and Gregorian calendars. Years are given in astronomical year numbering.
The adoption of the Gregorian Calendar has taken place in the history of most cultures and societies around the world, marking a change from one of various traditional dating systems to the contemporary system – the Gregorian calendar – which is widely used around the world today. Some states adopted the new calendar in 1582, others not before the early twentieth century, and others at various dates between. A few have yet to do so, but except for these, the Gregorian calendar is now the world's universal civil calendar, old style calendars remaining in use in religious or traditional contexts. During – and for some time after – the transition between systems, it has been common to use the terms "Old Style" and "New Style" when giving dates, to indicate which calendar was used to reckon them.
