A leap year is not every four years, but every four years except those not divisible by 400. This means that every year that is divisible by 100, but not by 400, is not a leap year. For example, the year 1900 was not a leap year, but the year 2000 was. This rule was introduced to keep our calendar in sync with the Earth's orbit around the Sun.The calendar is based on the solar year, which is the t...More