February 30
Let an agent explain February 30, relay why it does not normally exist, the Swedish 1712 case and Soviet calendar experiments and its use as an impossible date from calendar history sources, and distinguish it from February 29, leap years, other calendars and ordinary invalid dates.
Research draft, second pass
A second pass drafted this model: the structure a model of this thing needs, and what is known about it in the world. The line under this one says how the second half was obtained - researched against sources, or recalled without web access, in which case nothing here was read anywhere and every claim is a lead to verify. Unreviewed either way.
written by Claude from model knowledge without web access - no source was read, every claim is a lead to verify
Researched by: Claude
Purpose and description
Let an agent explain February 30, relay why it does not normally exist, the Swedish 1712 case and Soviet calendar experiments and its use as an impossible date from calendar history sources, and distinguish it from February 29, leap years, other calendars and ordinary invalid dates.
February 30 is a date that does not exist in the standard Gregorian or Julian calendars, since February has 28 days, or 29 in a leap year; it has appeared in a few historical and special cases, such as Sweden in 1712, when a botched calendar transition added a February 30 to realign its calendar, and in the Soviet revolutionary calendar experiments, and it is used as a deliberately impossible date in computing tests and fiction. It is a curiosity of calendar history rather than a real recurring date.
What it is for: A non-existent or historically exceptional calendar date.
It can be explain why it does not exist; relay the Swedish and Soviet cases; relay its use as an impossible date; distinguish related concepts.
Distinguishing features
Normally invalid
Rare historical cases
Calendar curiosity
Used as impossible date
What it looks like
Not a physical object; a date label that is normally invalid.
Physical character
Sweden: 30 February 1712 date - calendar realignment
February length: 28 or 29 days note
registry parent: 30th day of a month note
How it is recognised
A normally non-existent date, 30 February
February 30
February 29 exists in leap years; leap years add a day; other calendars have different months; ordinary invalid dates are simple errors
Related models
is a kind of - in registry terms, non-standard
is contrasted with -
is contrasted with -
is related to -
In practice
Families and kinds
the Swedish 1712 case
Soviet calendar experiments
impossible-date usage
Standards and regulation
Gregorian calendar rules
ISO 8601 date validity
Failure modes and hazards
Treating February 30 as a normal date
Date validation errors in software
Confusing calendar history
Where this came from
wikidata · CC0 1.0
Drafted structure
Bundle to layer to finding to question, as the second pass will find it: 4 bundles · 8 layers · 8 findings · 16 questions.
Understand What February 30 is.
Definition.
Definition
Definition.
Definition
Definition.
- What is February 30, and how does it differ from February 29, leap years, other calendars and ordinary invalid dates? definition
- Is the question about calendar history or software date validation? boundary
Cases
Historical cases.
Cases
Cases.
- What were the Swedish 1712 and Soviet calendar cases? definition
- Which entry fits calendar reform? action
Calendar Calendar rules.
Sources.
February
Length of February.
February
February.
- Why does February have 28 or 29 days? provenance
- Which references are standard? provenance
Leap
Leap years.
Leap
Leap.
- How do leap years add February 29? provenance
- Which entry fits leap year? action
History History.
Sources.
Sweden
Swedish 1712 case.
Sweden
Sweden.
- Why did Sweden have a 30 February in 1712? provenance
- Which sources are cited? provenance
Soviet
Soviet calendar.
Soviet
Soviet.
- How did Soviet calendar experiments relate to February 30? provenance
- Is the information accurate? boundary
Context Computing.
Regulation.
Validation
Date validation.
Validation
Validation.
- Why is February 30 used to test date validation? provenance
- Is the information current? boundary
Fiction
Fiction and jokes.
Fiction
Fiction.
- How is February 30 used as a deliberately impossible date? provenance
- Is the presentation neutral and attributed? boundary
What the second pass must settle
- How should historical exceptions be documented?
- How should date-validation edge cases be linked?
- Should the Swedish 1712 case be an instance?