23 hours 56 mins is a sidereal day. This is how long the earth takes to rotate against the background stars. However, as we're also orbiting the sun, over the course of a year, we "lose" a day, meaning that we experience a "solar day" (i.e. noon to noon) as an average of 24 hours. The length of a solar day changes by a few minutes throughout the year, but we just use the average on our clocks.