Calendars and time
Calendar Month
Direct answer
Calendar Month: A Calendar Month is a named subdivision of a calendar year whose length is fixed by that calendar rules.
A Calendar Month is a conventional named division of the year.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Tropical Month, Mean Solar Day, Calendar, Julian Date
A Calendar Month is a conventional named division of the year.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Calendar Month
Modern Gregorian month lengths preserve layers of Roman calendar development and reform.
Why Calendar Month matters
Calendar Month differs from an astronomical month because civil month lengths are whole-day conventions rather than exact lunar periods.
Calendar Month in practice: an example
Adding one calendar month to January 31 requires a stated rule because February has no January 31 counterpart.
Where readings of Calendar Month go wrong
It is a mistake to convert a calendar month to a fixed number of seconds without defining the start date and convention.
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.
- U.S. Naval Observatory: Astronomical Almanac Glossary - Reference definitions for astronomical periods, coordinates, dates, and time scales.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Check the month-length rule
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More in Calendars & Timekeeping
- Dominical Letter
- Equation of Time
- Intercalary Day
- Leap Second
- New Style Date
- Sidereal Month
- Tropical Year
- Anomalistic Year
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Common questions
What is a Calendar Month?
A Calendar Month is a named subdivision of a calendar year whose length is fixed by that calendar rules.
What do people get wrong about a Calendar Month?
It is a mistake to convert a calendar month to a fixed number of seconds without defining the start date and convention.
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