How do calendars divide time into days, months, and years?

Calendars divide time by using the movements of celestial bodies as natural standards, though these cycles are often incompatible, requiring mathematical adjustments to maintain accuracy.12

Days

The day is the fundamental unit of time, based on the Earth’s rotation. While a sidereal day measures the rotation relative to the stars, calendars typically use the mean solar day—a constant, averaged period of 24 hours—to ensure consistency for civil and scientific purposes.12

Months

The month is traditionally derived from the Moon’s cycle. The synodic month—the time between successive new moons—is approximately 29.53 days. Because this does not result in a whole number of days, calendars often alternate between 29 and 30 days to track these phases.23

Years

The year is based on the Earth’s orbit around the Sun. The tropical year (the time between successive vernal equinoxes) is approximately 365.2422 days. Because this is not a whole number, calendars use intercalation—the periodic addition of days or months—to keep the calendar in sync with the seasons. For example, the Gregorian calendar adds a leap day every four years, with specific exceptions for century years to account for the discrepancy between the calendar and the true tropical year.2435