Calendarmonth

Calendarmonth is a term used to describe the basic unit of time in civil calendars: the interval corresponding to a named month from its first day to its last day. In practice, a calendarmonth is a calendar month, which may contain 28, 29, 30, or 31 days depending on the month and leap-year rules. The concept is distinct from related measures such as lunar months or fiscal months, and is primarily used in civil timekeeping and date calculations.

In the Gregorian calendar, most calendarmonths have fixed lengths: January, March, May, July, August, October, and December have 31 days; April, June, September, and November have 30 days; February has 28 days in common years and 29 in leap years. The sequence of months and their lengths yields a 365-day common year or a 366-day leap year. Across cultures, calendarmonths can differ when other calendar systems are used.

Other calendars define calendarmonths with different lengths. The Islamic calendar uses lunar months of 29 or 30 days, and years drift relative to the solar year. Hebrew and other lunisolar calendars use 29- or 30-day months and may insert leap months to realign with the solar year. Some traditional calendars also adjust month lengths to fit astronomical or agricultural cycles.

In computing and standards, calendar-month arithmetic treats a calendarmonth as a unit; adding months can lead to variable day handling. For example, moving from January 31 by one calendar month often results in February 28 or 29, depending on the year and the system’s rules. The term emphasizes the civil, date-based unit rather than a fixed number of days.