calenders

A calendar is a system for organizing days for social, religious, commercial, and administrative purposes. It divides time into units such as years, months, and weeks and provides a framework for scheduling events and recording dates. Most calendars are built around cycles of the Sun, the Moon, or both.

Calendars can be solar, lunar, or lunisolar. Solar calendars approximate the tropical year and adjust with leap days. Lunar calendars track Moon phases and can drift relative to the seasons. Lunisolar calendars combine both cycles and insert leap months to realign with the solar year.

Historically, calendars have varied by culture. Ancient Egypt, Mesopotamia, and the Roman world developed early solar and lunisolar systems. The Julian calendar introduced a 365-day year with a leap day every four years. The Gregorian reform of 1582 refined the leap-year rule to better match the tropical year and is the basis of most civil calendars today.

Current examples include the Gregorian calendar for most of the world, the Iranian solar calendar, Islamic lunar calendars like the Hijri, and lunisolar systems such as the Hebrew and Chinese calendars. Many cultures also preserve ceremonial calendars tied to agricultural or religious cycles.

In modern usage, standardized date formats such as ISO 8601 (YYYY-MM-DD) facilitate international communication and computing. Calendar systems continue to guide timekeeping, scheduling, and historical dating across societies.