calendartype

Calendartype is a concept in calendrics that describes how a calendar system organizes the year. The three broad categories are solar calendars, which align with the Earth's orbit around the Sun; lunar calendars, which base months on the phases of the Moon; and lunisolar calendars, which combine lunar months with periodic adjustments to stay roughly synchronized with the solar year.

Solar calendars include the Gregorian calendar and many national civil calendars. They keep a nearly fixed number of days per year, using regular leap days or cycles to maintain alignment with the solar year. Lunar calendars, such as the Islamic calendar, base months on lunar cycles and do not by themselves align with the solar year. They drift relative to the seasons. Lunisolar calendars, including the Hebrew, Chinese, and many Hindu calendars, use lunar months but insert leap months or days to keep roughly in step with the solar year, producing years that vary in length and in the placement of leap months.

Calendar type influences date handling in software and in cultural practices, including how months are named, how holidays are determined, and how long terms are. Converting dates between types requires algorithms that account for different month lengths, intercalations, and era systems.

See also: calendar systems, chronology.