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List of non-standard dates
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 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 (Jan 0, 1900 is the same as Dec 31, 1899). January 0 also occurs in the epoch for the ephemeris second, "1900 January 0 at 12 hours ephemeris time". 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.
February 29 was a non-standard date until its adoption between the 15th and 17th centuries. Even afterward, it was not until the passage of the Calendar (New Style) Act 1750 that 29 February was formally recognised in British law. This additional date in February was added to avoid doubling February 24th in the Julian calendar to mark a leap-year.
February 30 is a day 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.
February 30 was a day that happened in Sweden in 1712. 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, so that the next leap year after 1696 would not be until 1744. Although the leap day was omitted in February 1700, the Great Northern War had begun, 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.
To avoid confusion and further mistakes, the Julian calendar was restored in 1712 by adding a second 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. The Swedish conversion to the Gregorian calendar was finally accomplished in 1753, when February 17 was followed by March 1.
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.
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List of non-standard dates AI simulator
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List of non-standard dates
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 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 (Jan 0, 1900 is the same as Dec 31, 1899). January 0 also occurs in the epoch for the ephemeris second, "1900 January 0 at 12 hours ephemeris time". 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.
February 29 was a non-standard date until its adoption between the 15th and 17th centuries. Even afterward, it was not until the passage of the Calendar (New Style) Act 1750 that 29 February was formally recognised in British law. This additional date in February was added to avoid doubling February 24th in the Julian calendar to mark a leap-year.
February 30 is a day 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.
February 30 was a day that happened in Sweden in 1712. 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, so that the next leap year after 1696 would not be until 1744. Although the leap day was omitted in February 1700, the Great Northern War had begun, 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.
To avoid confusion and further mistakes, the Julian calendar was restored in 1712 by adding a second 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. The Swedish conversion to the Gregorian calendar was finally accomplished in 1753, when February 17 was followed by March 1.
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.