List of non-standard dates

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There are several non-standard dates that 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.

December 31 is the 365th day of the year in the Gregorian calendar. It is known by a collection of names including: Saint Sylvester's Day, New Year's Eve or Old Years Day/Night, as the following day is New Year's Day. It is the last day of the year. The following day is January 1 of the following year.

In an ephemeris

January 0 refers to 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.

In astronomy and celestial navigation, an ephemeris gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position over time. The etymology is from Latin ephemeris, meaning 'diary' and from Greek, Modern εφημερίς (ephemeris), meaning 'diary, journal'. Historically, positions were given as printed tables of values, given at regular intervals of date and time. The calculation of these tables was one of the first applications of mechanical computers. Modern ephemerides are often computed electronically, from mathematical models of the motion of astronomical objects and the Earth. However, printed ephemerides are still produced, as they are useful when computational devices are not available.

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 astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as the celestial coordinates or elliptical orbital elements of a celestial body, because these are subject to perturbations and vary with time. These time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, the node of its orbit relative to a reference plane, the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit.

Newcomb's Tables of the Sun is the short title and running head of a work by the American astronomer and mathematician Simon Newcomb entitled "Tables of the Motion of the Earth on its Axis and Around the Sun" on pages 1–169 of "Tables of the Four Inner Planets" (1895), volume VI of the serial publication Astronomical Papers Prepared for the Use of the American Ephemeris and Nautical Almanac. The work contains Newcomb's mathematical development of the position of the Earth in the Solar System, which is constructed from classical celestial mechanics as well as centuries of astronomical measurements. The bulk of the work, however, is a collection of tabulated precomputed values that provide the position of the sun at any point in time.

In software

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

Microsoft Excel Spreadsheet editor, part of Microsoft Office

Microsoft Excel is a spreadsheet developed by Microsoft for Windows, macOS, Android and iOS. It features calculation, graphing tools, pivot tables, and a macro programming language called Visual Basic for Applications. It has been a very widely applied spreadsheet for these platforms, especially since version 5 in 1993, and it has replaced Lotus 1-2-3 as the industry standard for spreadsheets. Excel forms part of the Microsoft Office suite of software.

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.

The Gregorian calendar is the calendar used in most of the world. It is named after Pope Gregory XIII, who introduced it in October 1582. The calendar spaces leap years to make the average year 365.2425 days long, approximating the 365.2422-day tropical year that is determined by the Earth's revolution around the Sun. The rule for leap years is:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.

February is the second and shortest month of the year in the Julian and Gregorian calendar with 28 days in common years and 29 days in leap years, with the quadrennial 29th day being called the leap day. It is the first of five months to have a length of fewer than 31 days, and the only month to have a length of fewer than 30 days, with the other seven months having 31 days. In 2019, February had 28 days.

A leap year is a calendar year containing one additional day added to keep the calendar year synchronized with the astronomical or seasonal year. Because seasons and astronomical events 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 called a common year.

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]

The Julian calendar, proposed by Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on 1 January 45 BC, by edict. It was the predominant calendar in the Roman world, most of Europe, and in European settlements in the Americas and elsewhere, until it was refined and gradually replaced by the Gregorian calendar, promulgated in 1582 by Pope Gregory XIII.

Swedish Empire the years 1611–1721 in the history of Sweden

The Swedish Empire was a European great power that exercised territorial control over much of the Baltic region during the 17th and early 18th centuries. The beginning of the Empire is usually taken as the reign of Gustavus Adolphus, who ascended the throne in 1611, and its end as the loss of territories in 1721 following the Great Northern War.

Great Northern War Conflict between mainly the Swedish and Russian empires in 1700–1721

The Great Northern War (1700–1721) was a conflict in which a coalition led by the Tsardom of Russia successfully contested the supremacy of the Swedish Empire in Northern, Central and Eastern Europe. The initial leaders of the anti-Swedish alliance were Peter I of Russia, Frederick IV of Denmark–Norway and Augustus II the Strong of Saxony–Poland–Lithuania. Frederick IV and Augustus II were defeated by Sweden, under Charles XII, and forced out of the alliance in 1700 and 1706 respectively, but rejoined it in 1709 after the defeat of Charles XII at the Battle of Poltava. George I of Great Britain and of Brunswick-Lüneburg (Hanover) joined the coalition in 1714 for Hanover and in 1717 for Britain, and Frederick William I of Brandenburg-Prussia joined it in 1715.

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]

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 or 31-day February, while the Symmetry454 calendar contains a 35-day February.

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 J. R. R. Tolkien's Middle-earth legendarium, the Hobbits have developed the Shire Reckoning. According to Appendix D to The Lord of the Rings , this calendar has arranged the year neatly in 12 months of 30 days each, together with holidays that are assigned to no month. 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 traditional calendar 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.

February 31

February 31 or 31 February is exceptionally used on gravestones when the date is unknown, [11] or in one case out of superstition. [12]

It is also used (along with February 32 and February 33) for calculating weather data. [13]

March 0

March 0 or 0 March is an alternative name for the last day of February as a way of allowing for the fact that its number of days varies in leap years. It is used most often in astronomy, software engineering, [14] 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. [15] 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 .

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. It can also be used similar to January 0 to signify the first day of the following year.[ citation needed ]

See also

Related Research Articles

A calendar is a system of organizing days for social, religious, commercial or administrative purposes. 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.

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 both days and months.

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. The Revised Julian calendar temporarily aligns its dates with the Gregorian calendar 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, Poland, and Romania.

Year Orbital period of the Earth around the Sun

A year is the orbital period of 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. The current year is 2019.

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 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.

Computus is a calculation that determines the calendar date of Easter. Because the date is based on a calendar-dependent equinox rather than the astronomical one, there are differences between calculations done according to the Julian calendar and the modern Gregorian calendar. The name has been used for this procedure since the early Middle Ages, as it was considered the most important computation of the age.

Swedish calendar used in Sweden and Finland between 1700 and 1712

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.

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.

Perpetual calendar calendar valid for many years

A perpetual calendar is a calendar valid for many years, usually designed to allow the calculation of the day of the week for a given date in the future.

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 some centuries ago, 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 or Eritrean calendar is the principal calendar used in Ethiopia and also serves as the liturgical year for Christians in Eritrea and Ethiopia belonging to the Eritrean Orthodox Tewahedo Church, Ethiopian Orthodox Tewahedo Church, Eastern Catholic Churches, the Coptic Orthodox Church of Alexandria, and Ethiopian-Eritrean Evangelicalism. It is a solar calendar which in turn derives from the Egyptian 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 7–8 years between the Ethiopian and Gregorian calendars results from an alternative calculation in determining the date of the Annunciation.

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 nations and societies, marking a change from their traditional dating system to the modern dating system that is widely used around the world today. Some countries 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 2012-03-12. Retrieved 2006-12-31.
  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 21 August 2016. Retrieved 7 November 2013.
  5. "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". 22 November 2005. Archived from the original on 22 November 2005. Retrieved 21 March 2017.CS1 maint: BOT: original-url status unknown (link)
  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". 6 June 2012. Retrieved 21 March 2017.
  12. Troy Taylor (2005). Weird Illinois. Sterling Publishing Co., Inc. p. 212.
  13. J. D. Everett (1863). "Description of a method of Reducing Observations of Temperature". The American Journal of Science and Arts. S. Converse: 23.
  14. "China tightens information controls for Tiananmen anniversary". The Age. Australia. Agence France-Presse. June 4, 2009. Retrieved November 3, 2010.