Calendars and time
Intercalation
Direct answer
Intercalation is the insertion of an extra day, month, or other interval to keep a calendar aligned with its astronomical or seasonal target.
Intercalation adds calendar time to correct accumulated drift.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Week Number, Intercalary Day, Calendar, Julian Date
Intercalation adds calendar time to correct accumulated drift.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Intercalation
Ancient administrators and astronomer-priests used observation or fixed cycles to decide when an extra interval was required.
Why Intercalation matters
Without intercalation, calendars based on incommensurable lunar months, solar years, and whole days gradually move away from their intended season or phase.
Intercalation in practice: an example
Gregorian intercalation adds February 29 in most years divisible by four, with century exceptions.
Where readings of Intercalation go wrong
It is a mistake to assume every intercalation rule follows the Gregorian four-year pattern.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Introduction to Calendars - Official overview of calendar systems, leap rules, eras, and historical calendar reform.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
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More in Calendars & Timekeeping
- Local Apparent Solar Time
- Old Style Date
- Universal Time
- Astronomical Day
- Callippic Cycle
- Draconic Month
- Geocentric Coordinate Time
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Common questions
What is an intercalation?
Intercalation is the insertion of an extra day, month, or other interval to keep a calendar aligned with its astronomical or seasonal target.
What do people get wrong about an intercalation?
It is a mistake to assume every intercalation rule follows the Gregorian four-year pattern.
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