leap year
Let an agent compute leap years correctly across calendars and handle 29 February in dates, ages and software.
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 compute leap years correctly across calendars and handle 29 February in dates, ages and software.
A calendar year containing an extra day (or month in lunisolar calendars) to keep the calendar aligned with the astronomical year; in the Gregorian calendar, years divisible by 4 are leap years except century years not divisible by 400, and the extra day is 29 February.
What it is for: Keeping calendars in step with the seasons.
It can be check whether a year is a leap year; handle 29 February in software; compute ages and anniversaries; convert between calendars.
Distinguishing features
Extra intercalary day
Gregorian rule with century exception
Julian rule is every 4 years
Leap seconds are a separate concept
What it looks like
A calendar with 366 days, including 29 February.
Physical character
length: 366 days - Gregorian
frequency: 97 in 400 years - Gregorian
How it is recognised
Year with 29 February
Divisibility rule
Leap years by starting weekday are calendar variants
Related models
is a kind of - category
is defined by - calendar
has part - leap day
is confused with - another concept
In practice
Families and kinds
Gregorian leap years
Julian leap years
century leap years
lunisolar leap months
leap years by starting weekday
Standards and regulation
ISO 8601 date representation
Laws on 29 February birthdays for legal age in some countries
Failure modes and hazards
Software bugs on 29 February
Wrong century rule
Age calculations for leap-day births
Also called
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.
Rule Is it a leap year.
Rules differ by calendar.
Gregorian
Divisibility.
Gregorian
Gregorian rule.
- Is this year a leap year under the Gregorian rule? boundary
- Is it a century year? boundary
Other calendars
Julian, lunisolar.
Other
Other calendars.
- Under which calendar is the question asked? definition
- What rule does that calendar use? definition
Dates Calendar arithmetic.
Leap days affect arithmetic.
Day count
Durations.
Day count
Day counting.
- How many days are between these dates? measurement
- Is a leap day included? boundary
Weekday
Starting day.
Weekday
Weekday.
- On which weekday does the year start and end? measurement
- Which calendar pattern repeats? definition
Software Bugs.
Leap days break software.
Handling
Libraries.
Handling
Date handling.
- Does the code use a tested date library? boundary
- Is 29 February tested? boundary
Adding years
Edge cases.
Adding years
Adding years.
- What happens when a year is added to 29 February? boundary
- Which convention applies? definition
People Leap-day births.
Birthdays need rules.
Birthdays
Celebration.
Birthdays
Leap-day birthdays.
- When does someone born on 29 February celebrate in common years? definition
- What does law say about their legal age? provenance
Records
Forms.
Records
Records.
- Do forms and systems accept 29 February? boundary
- Who fixes them? action
What the second pass must settle
- Should leap years by weekday be separate entries?
- How should calendar conversions be linked?
- How should leap-day legal rules be recorded?