List of non-standard dates
description of non-standard dates used in calendars for various reasons
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.
Historical
January 0
January 0 is an alternative date for December 31. January 0 is the day before January 1 and after December 30 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
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.
Before this change, February 24 was doubled in leap years in the Julian calendar.
February 30
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.
“List of non-standard dates” enters the record as description of non-standard dates used in calendars for various reasons. Crown Archives preserves that source wording while asking what List, non-standard and dates can confirm, complicate or overturn.
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The citation trail is more important than the brevity of the summary: it shows where individual claims can be examined in context. The source revision retrieved here is dated Sep 20, 2026. The linked authority identifier is Q60674392. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1900, 1899 and 1750.
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This entry incorporates text from “List of non-standard dates” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.