History
How do calendars measure time?
Days follow Earth’s rotation, months echo lunar cycles and years track Earth’s orbit—then rules make them fit.
Three natural cycles do not divide evenly
The solar day, lunar phase cycle and tropical year are independent astronomical periods. A year is not a whole number of lunar months or days, so every calendar must choose compromises and correction rules.
Leap rules keep seasons aligned
The Gregorian calendar normally has 365 days and inserts leap days to approximate the tropical year. Century years are leap years only when divisible by 400, preventing the calendar from drifting as quickly as the older Julian rule.
Calendars answer different cultural needs
Solar calendars keep months aligned with seasons, lunar calendars follow Moon phases, and lunisolar calendars add intercalary months to reconcile both. Civil date conversion therefore depends on which calendar system is being used.