Calendars and time
Leap Year
Direct answer
Leap Year: A Leap Year is a calendar year with an added day or other intercalation used to keep calendar cycles aligned with astronomical or seasonal cycles.
A Leap Year adds calendar time to correct accumulated mismatch with a natural cycle.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Calendar, Solar Calendar, Lunar Calendar, Lunisolar Calendar
A Leap Year adds calendar time to correct accumulated mismatch with a natural cycle.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Leap Year
Intercalation predates modern calendars; the Julian and Gregorian reforms use different leap-day patterns.
Why Leap Year matters
The solar year is not an exact whole number of days, so a civil calendar needs a correction rule to limit long-term drift.
Leap Year in practice: an example
Gregorian century years are leap years only when divisible by 400.
Where readings of Leap Year go wrong
A common error is to apply the Gregorian rule automatically to historical dates recorded in another calendar.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Introduction to Calendars - Calendar definitions and the astronomical basis of solar, lunar, lunisolar, Julian, and Gregorian systems.
- U.S. Naval Observatory: Astronomical Almanac Glossary - Reference definitions for astronomical coordinates, events, calendars, and time scales.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Check a calendar's leap rule
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More in Calendars & Timekeeping
- New Style Date
- Sidereal Month
- Tropical Year
- Anomalistic Year
- Calendar Year
- Double Dating
- Era
- Intercalary Month
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Common questions
What is a Leap Year?
A Leap Year is a calendar year with an added day or other intercalation used to keep calendar cycles aligned with astronomical or seasonal cycles.
What do people get wrong about a Leap Year?
A common error is to apply the Gregorian rule automatically to historical dates recorded in another calendar.
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