List of non-standard dates

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Several non-standard dates are used in calendars. Some are used sarcastically, some for scientific or mathematical purposes, and some for exceptional or fictional calendars.


January 0

January 0 or 0 January is an alternative name for December 31.

In an ephemeris

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]

In software

In Microsoft Excel, the epoch of the 1900 date format is January 0, 1900. [3]

February 30

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. February 30 is usually used as a sarcastic date for referring to something that will never happen or will never be done. [4] However, this date did happen once on the Swedish calendar in 1712. [5] It also appears in some reform calendars.

Swedish calendar

Swedish calendar for February 1712 Feb1712.jpg
Swedish calendar for February 1712

February 30 was a real date in Sweden in 1712. [5]

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. [6]

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. [6] [7]

The Swedish conversion to the Gregorian calendar was finally accomplished in 1753, when February 17 was followed by March 1. [6]

Artificial calendars

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. [8]

Fictional calendars

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. [9]

February 30, 1951, is the last night of the world in Ray Bradbury's short story "Last Night of the World". [10]


Soviet calendar

Although many sources erroneously state that 30-day months were used in the Soviet Union for part or all of the period from 1929 to 1940, the Soviet calendar with 5- and 6-day weeks was used only for assigning workdays and days of rest in factories. The Gregorian calendar (since 1918) remained for everyday use: surviving physical calendars from that period show only the irregular months of the Gregorian calendar, including a 28- or 29-day February, so there was never a February 30 in the Soviet Union.

Early Julian calendar

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. [11]

February 31

February 31 or 31 February is exceptionally used on gravestones when the date is unknown, [12] [ better source needed ] or in one case out of superstition. [13]

It is also used (along with February 32 and February 33) for calculating weather data. [14]

March 0

March 0 or 0 March is an alternative name for the last day of February (February 28, or February 29 in leap years). It is used most often in astronomy, software engineering, [15] and Doomsday Algorithm calculations.

May 35

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 Communist government, and the event is normally referred to as June 4. [16] 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.

June 31

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

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.

December 32, 1980

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 the date was officially recorded by sympathetic British government officials as "December 32, 1980". [17]

Reform calendars

Because evening out the months is a 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.

See also

Related Research Articles

Calendar A 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, 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.

The Julian calendar, proposed by Julius Caesar in AUC 708, was a reform of the Roman calendar. It took effect on 1 January AUC 709 , by edict. It was designed with the aid of Greek mathematicians and Greek astronomers such as Sosigenes of Alexandria.

A leap year is a calendar year that contains an additional day added to keep the calendar year synchronized with the astronomical year or seasonal year. Because astronomical events and seasons do not repeat in a whole number of days, calendars that have the same number of days in each year drift over time with respect to the event that the year is supposed to track. By inserting an additional day or month into the year, the drift can be corrected. A year that is not a leap year is a common year.

The Revised Julian calendar, also known as the Milanković calendar, or, less formally, new calendar, is a calendar proposed by the Serbian scientist Milutin Milanković in 1923, which effectively discontinued the 340 years of divergence between the naming of dates sanctioned by those Eastern Orthodox churches adopting it and the Gregorian calendar that has come to predominate worldwide. This calendar was intended to replace the ecclesiastical calendar based on the Julian calendar hitherto in use by all of the Eastern Orthodox Church. From 1 March 1600 through 28 February 2800, the Revised Julian calendar aligns its dates with the Gregorian calendar, which was proclaimed in 1582 by Pope Gregory XIII for adoption by the Christian world. The calendar has been adopted by the Orthodox churches of Constantinople, Albania, Alexandria, Antioch, Bulgaria, Cyprus, Greece, and Romania.

Year Orbital period of the Earth around the Sun

A year is the orbital period of a planetary body, for example, the Earth, moving in 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 hours of daylight, and, consequently, vegetation and soil fertility. In temperate and subpolar regions around the planet, four seasons are generally recognized: spring, summer, autumn and winter. In tropical and subtropical regions, several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked.

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.

Coptic calendar Egyptian liturgical calendar

The Coptic calendar, also called the Alexandrian calendar, is a liturgical calendar used by the Coptic Orthodox Church and also used by the farming populace in Egypt. 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 the intercalation of a sixth epagomenal 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.

The Soviet calendar refers to the Gregorian calendar implemented in 1918, national holidays, and five- and six-day work weeks used between 1929 and 1940. The Gregorian calendar, under the name "Western European calendar", was implemented in Soviet Russia in February 1918 by dropping the Julian dates of 1–13 February 1918. As many as nine national holidays were implemented in the following decade, but four were eliminated or merged on 24 September 1929, leaving only five national holidays, 22 January, 1–2 May, and 7–8 November, to celebrate until 1951, when 22 January reverted to a normal day. During 1929 to 1940, five- and six-day work weeks were used to schedule work, but the Gregorian calendar and its seven-day week were used for all other purposes.

The computus is a calculation that determines the calendar date of Easter. Easter is traditionally celebrated on the first Sunday after the Paschal full moon, which is the first full moon on or after 21 March. 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 calendar. The calculations produce different results depending on whether the Julian calendar or the Gregorian calendar is used.

Swedish calendar

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 nominally astronomically from 1740 to 1844.

The Darian calendar is a proposed system of timekeeping designed to serve the needs of any possible future human settlers on the planet Mars. It was created by aerospace engineer, political scientist, and space jurist Thomas Gangale in 1985 and named by him after his son Darius. It was first published in June 1986. In 1998 at the founding convention of the Mars Society the calendar was presented as one of two calendar options to be considered along with eighteen other factors to consider for the colonization of Mars.

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 on.

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.

The Ethiopian calendar is the principal calendar used in Ethiopia and also serves as the liturgical year for Christians in Ethiopia and Eritrea belonging to the Ethiopian Orthodox Tewahedo Church, Eritrean Orthodox Tewahedo Church, Eastern Catholic Churches, the Coptic Orthodox Church of Alexandria, and P'ent'ay Churches. The Ethiopian calendar is a solar calendar that has more in common with the coptic calendar, but like the Julian calendar, it adds a leap day every four years without exception, and begins the year on August 29 or August 30 in the Julian 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 Symmetry454 calendar (Sym454) is a proposal for calendar reform created by Irv Bromberg of the University of Toronto, Canada. It is a perennial solar calendar that conserves the traditional month pattern and 7-day week, has symmetrical equal quarters in 82% of the years in its 293-year cycle, and starts every month on Monday.

The Gregorian calendar is the calendar used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a minor modification of the Julian calendar, reducing the average year from 365.25 days to 365.2425 days, thus correcting for the drift against the solar year that the inaccuracy had caused during the intervening centuries.

Conversion between Julian and Gregorian calendars

The tables below list equivalent dates in the Julian and Gregorian calendars. Years are given in astronomical year numbering.

Adoption of the Gregorian calendar

The adoption of the Gregorian Calendar was an event in the modern history of most cultures and societies, marking a change from their traditional dating system to the modern dating system that is widely used around the world today. Some states adopted the new calendar from 1582, some did not do so before the early twentieth century, and others did so at various dates between; however a number continue to use a different civil calendar. For many the new style calendar is only used for civil purposes and the old style calendar remains used in religious contexts. Today, the Gregorian calendar is the world's most widely used civil calendar. During – and for some time after – the change 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.


  1. "Leap Seconds". Time Service Department, United States Naval Observatory. Archived from the original on February 28, 2012. Retrieved December 31, 2006.
  2. Ransom, Jr., David H. (November 19, 1989). "Program ASTROCLK: Astronomical Clock and Celestial Tracking Program with Celestial Navigation". p. 110.
  3. Lowe, Scott (May 11, 2007). "How do I... Perform basic formatting in Excel 2003?". TechRepublic .
  4. "Thirty Days Hath February 2000?". Archived from the original on August 21, 2016. Retrieved November 7, 2013.
  5. 1 2 "February 30 Was a Real Date". Retrieved April 25, 2016.
  6. 1 2 3 Bauer, R. W. (1868). Calender for Aarene fra 601 til 2200. Copenhagen, Denmark: Dansk Historisk Fællesråd (1993 reprint). p. 100. ISBN   87-7423-083-2.
  7. Vallerius, Johannes (1711). Allmanach på åhret effter Christi födelse 1712. Lund, Sweden.
  8. "Hadley Centre: GDT netCDF conventions". November 22, 2005. Archived from the original on November 22, 2005. Retrieved March 21, 2017.
  9. Tolkien, J. R. R. (1965). "Appendix D". The Return of the King (2nd ed.). Boston: Houghton Mifflin Co. ISBN   978-0-395-08256-0.
  10. "A Classic Ray Bradbury Esquire Story". June 6, 2012. Retrieved March 21, 2017.
  11. Roscoe Lamont, "The Roman calendar and its reformation by Julius Caesar", Popular Astronomy 27 (1919) 583–595. Sacrobosco's theory is discussed on pages 585–587.
  12. "February 31 On Gravestone". Swampy Acres Farm Blog. December 19, 2018. Archived from the original on December 19, 2018. Retrieved June 13, 2019.
  13. Troy Taylor (2005). Weird Illinois. Sterling Publishing Co., Inc. p. 212.
  14. J. D. Everett (1863). "Description of a method of Reducing Observations of Temperature". The American Journal of Science and Arts. S. Converse: 23.
  15. "China tightens information controls for Tiananmen anniversary". The Age. Australia. Agence France-Presse. June 4, 2009. Retrieved November 3, 2010.
  16. "Lear Fan 2100 (Futura)". The Museum of Flight. 2009. Archived from the original on July 12, 2009. Retrieved November 27, 2009.