Equation of time: Not every day is the same length. Due to the elliptical orbit of the earth around the sun and the inclination of the earth's axis, there are actually only that there are actually only 4 days in the year on which the day is exactly 24 hours long (April 15, June 14, September 1 and December 24). All other days are a little shorter or longer. This deviation is up to +/- 15 minutes per day. This is indicated by the equation of time on the "4:30". The equation of time display is linked to the calendar displays and does not need to be set separately.