What is leap day exactly? Temple math professor Yury Grabovsky says Leap Day exists because it actually takes about 365 and a quarter days for the earth to revolve around the sun. "After four years, you get a day error, after 40 years, you get 10 days error. So as you can see, the error adds up over time." He says Leap Day helps to correct the error and ensures that the seasons start when expected.