Historical: Since the dawn of civilization humans have tried to organize the day by the motion of the sun, moon, and stars. Shadow clocks, later named sundials, were invented by Greeks and Romans and used by explorers during our age of discovery. The sundial played a fundamental role in finding local time. Watches were notoriously unreliable and often needed to be reset by using sun-time. And up till the 20th C. each locality had its own miniature time zone, often differing in minutes only. By pointing the triangular style (gnomon) with the number 12 due North, the shadow of the style will point at the sunny hour.

Info: Sunny Hours