Leap Years are needed to keep our modern day Gregorian calendar in alignment with the Earth’s revolutions around the sun. It takes the Earth approximately 365.242199 days – or 365 days, 5 hours, 48 minutes, and 46 seconds – to circle once around the Sun. This is called a tropical year.However, the Gregorian calendar has only 365 days in a year, so if we didn’t add a day on February 29 nearly every 4 years, we would lose almost six hours off our calendar every year. After only 100 years, our calendar would be off by approximately 24 days! This happened in the Julian calendar, which was in use until the Gregorian was introduced.