In a metronomic whiteboard animation, Minute Physics explains how a solar day differs from a standard calendar day. A solar day encompasses a full rotation of the Earth’s axis, which is both elliptical and tilted. Keeping track of solar days would require far more maintenance than the accurate, yet arbitrary 24-hour mean calendar day that’s used the world over.
This discrepancy is due to the complications of the earth’s orbit being elliptical and the earth’s spin axis being tilted. If we used solar days in everyday life, we’d either need to have calendars and clocks that changed the number of minutes and seconds in a day depending on the time of year, or we’d need to have clocks that changed the length of a second (or changed the number of seconds in an hour) depending on the time of year.