A leap year is a calendar adjustment implemented to synchronize human timekeeping with the Earth's orbital period around the Sun. Due to the planet's elliptical path, one full orbit requires slightly more time than the standard 365-day calendar year. To account for the accumulating fractional difference, a leap year adds an extra day, typically making the calendar year 366 days long. The precise rules for this correction involve complex cycles; while a simple rule dictates adding a day every four years, further rules exist to correct for the cumulative error across centuries, often involving exceptions for years divisible by 100 and exceptions to those exceptions for years divisible by 400.