Kalender

Kalender, in many languages including Dutch, denotes a system for organizing time into units such as days, weeks, months, and years. It is used to schedule events, plan seasonal activities, and coordinate social life. The word calendar derives from the Latin calendarium, meaning a register or accounts book.

Calendars are classified by how they measure time: solar calendars track the cycle of the Sun, lunar calendars follow the Moon’s phases, and lunisolar calendars try to reconcile both cycles by inserting leap months. Solar calendars aim to keep seasons roughly fixed; lunar calendars drift relative to the seasons unless adjusted. Most of the world uses a solar calendar today.

Historically, many traditions created calendars adapted to their environment. Ancient Egypt and Mesopotamia developed early solar and lunisolar systems; the Roman calendar underwent reforms resulting in the Julian calendar and later the Gregorian reform in 1582. Numerous cultural calendars persist, including the Islamic calendar (lunar), the Hebrew calendar (lunisolar), the Hindu calendars (solar and lunisolar), and the Chinese calendar (lunisolar).

Today, the Gregorian calendar is the de facto civil calendar globally, with 12 months and a 4-year leap cycle. Week structure and the names of months vary by culture, but most systems divide time into weeks of seven days. In addition to civil use, calendars regulate religious feasts, agricultural cycles, and academic terms, and digital calendars enable scheduling across devices and time zones.