Since 45 B.C., leap years have been used to reconcile calendars with the actual time it takes for the earth to circle the sun. It takes the earth 365 days, 5 hours, 48 minutes and 45 seconds to revolve around the sun, not 365 days. If left uncorrected, calendar dates and important events, such as the vernal and autumnal equinoxes and the solstices, will slowly shift until they don’t match their prescribed calendar dates. After a century, our calendar would be off by about 24 days.