Leap years exist because Earth refuses neat arithmetic
A year is not exactly 365 days, so calendars insert extra days—or months—to stop seasons from sliding. The Gregorian rule skipping three century leap years in every four centuries is a precision trick Julius Caesar never needed.
Calendars with fixed day counts inevitably drift against astronomical seasons because Earth's orbit lasts slightly less than 365¼ days. Leap years add an extra day—or, in lunisolar systems, an extra month—to resynchronize civil time with the solar year. Without correction, planting festivals and equinoxes would wander through the calendar.
Julius Caesar's 45 BC reform made Rome's calendar consistently solar: add one leap day every four years. A Julian year averages 365.25 days versus a tropical year of about 365.2422 days—only eleven minutes off, but enough to drift three days every four centuries. February was left short because Roman tradition treated even-numbered months as unlucky.
The Gregorian refinement keeps March equinox near 21 March for Easter calculations. Years divisible by four leap, yet century years leap only when divisible by 400—so 1600 and 2000 leap, but 1700, 1800, and 1900 do not. Over 400 years the cycle totals 146,097 days, averaging 365.2425 days. That trims three excess leap days per four centuries compared with Julian rules.
The term "leap year" may reflect how weekdays jump: after a 29 February, dates from 1 March through the next 28 February advance two weekday slots instead of one. Romans first doubled 24 February—the bissextile—treating both sixth-days-before-March as one legal day. England counted the leap day as nonexistent until the Calendar Act of 1750 formally recognized 29 February. Leap seconds in UTC are a separate, irregular correction for Earth's rotation.
Source: Leap year
