kalenders

Kalenders are systems for organizing days into repeating units such as years, months, and weeks. They provide a shared framework for civil life, agriculture, religion, and commerce by aligning social activities with the cycle of time. A calendar defines how time is counted, how days are numbered, and when units such as months or leap days appear.

Most calendars fall into solar, lunar, or lunisolar categories. Solar calendars are based on the solar year, the time the Earth takes to orbit the Sun. Lunar calendars follow lunar months, roughly 29.5 days each. Lunisolar calendars try to reconcile both cycles, keeping months in step with the seasons by adding leap months or days as needed. Some calendars are sidereal, tied to the positions of the stars rather than the Sun.

Calendars have ancient origins in civilizations such as Mesopotamia, Egypt, and the Indus Valley. The Roman Julian calendar introduced a solar year with regular leap years, and the later Gregorian reform corrected drift relative to the solar year; today it underpins most civil timekeeping. Other calendars persist for cultural or religious reasons, including the Hebrew calendar, the Islamic Hijri calendar, various Hindu calendars, the Chinese calendar, and the Persian solar calendar.

In contemporary use, calendar systems coexist. The Gregorian calendar is dominant for civil purposes, while many cultures observe traditional calendars to determine holidays and rituals. Dates are often standardized for international communication, with formats and time zones supporting cross-border planning and record-keeping.