A common year is a calendar year with 365 days, as distinguished from a leap year, which has 366 days. More generally, a common year is one without intercalation. The Gregorian calendar, used by the majority of the world, employs both common years and leap years. This is to keep the calendar aligned with the tropical year, which does not contain an exact number of days. A common year is approximately a quarter day shorter than a tropical year, which has 365.24 days. If the Gregorian calendar only used common years and omitted leap years, the calendar would be out of sync with the tropical year by approximately 24 days in 100 years.