Sun-dials were the only time-keepers known for centuries before clocks and watches were invented. Sun-dial time is called Apparent time, and clock time Mean time, and the difference between the two is known as the equation of time. A sun-dial and a clock will not agree absolutely except at four different periods of the year, for a few days in the middle of June, the middle of September, December and March. At these periods we have the typical days of the year, the longest days in June, the shortest in December, and the twelve-hour day in September and March when the sun rises and sets at six o’clock. At all other times the clock is either a few minutes faster or a few minutes later than the Apparent time. So you don’t want to “set your clock by it”, but the sun-dial has other ornamental value.