The oldest clock made by humans was a sundial that indicated time by the length or angle of the shadow of a stick cast by the sun. Sundials were first used in about 4000 B.C. in Egypt and spread to various parts of the world. A sundial in those years was simply a vertical stick thrust into the ground. In the Northern Hemisphere, this shadow traversed to the right, or clockwise. Since major civilizations at the time clocks were invented were concentrated in the Northern Hemisphere, the hands of clocks and watches rotate to the right.
A system that divides the day into 100, 12, or other equal time lengths is called an equal hours system. Currently, one day is divided into 24 equal time lengths. An unequal hours system divides the day into day and night, each of which is then divided into equal intervals. Since the lengths of day and night change with the seasons, the lengths of these intervals also change. In the Edo Period, the dawn (Akemutsu) and the dusk (Kuremutsu) were set as reference points that divided the day into day and night. Day and night were then each divided into six equal intervals. The length of each interval differed for days and nights and varied with the seasons.
On June 10, 671, Emperor Tenji introduced a water clock called Rokoku, the first instrument in Japan to measure time and indicate hours using the sound of a bell or drum. In 1920, Seikatsu Kaizen Domei-kai (The Life Improvement Union) designated June 10 Time Memorial Day to highlight the importance of time as part of efforts to improve life.