kalendere

Kalendere, commonly referred to as calendars in English, are systems for partitioning time into cycles such as days, weeks, months, and years. They standardize the sequence of days, determine when to observe holidays, and organize civil, religious, and agricultural activities. Calendars can be solar, lunar, or lunisolar in their basis.

Ancient calendars often followed lunar cycles; later societies developed solar or lunisolar systems. The Julian calendar of 45 BCE introduced a 365-day year with leap years. The Gregorian reform of 1582 adjusted drift by omitting leap years in century years unless divisible by 400; adoption spread gradually across regions.

Types: Solar calendars fix a year length near 365 days and insert leap days as needed (Gregorian, Julian). Lunar calendars place months according to lunar cycles, resulting in years shorter than solar years (Islamic calendar). Lunisolar calendars use both cycles and insert intercalary months to re-align months with the solar year (Hebrew, Chinese, Hindu calendars).

Most modern usage relies on the Gregorian calendar for civil life, with ISO week dates and year-week numbering used in business and computing. Digital calendars use standardized formats such as ICS to store events. Various cultures retain religious or agricultural calendars alongside the civil one.

Kalendere have influenced timekeeping, taxation, farming, and ritual life. Calendar reforms have shaped political authority and cultural identity, while global communication increased synchronization among different systems.