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.

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.

[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.

[15] The LearAvia Lear Fan aircraft test flight had British government "funding that expired at the end of that year."

by Dr. Seuss narrates many delightful things which are supposed to happen starting on the first day of the fictional month of Octember.

[25] 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.

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.

The Symmetry454 calendar assigns 35 days to February, May, August, and November, as well as December in a leap year.