The sand glass was one of the earliest methods by which man measured time. The invention of the hourglass is credited to a monk by the name of Luitprand from Chartres, France; sometime around the turn of 8th century.
Charlemagne is said to have had one that was so large that it had to be turned just once every twelve hours. He had divisions marked on the outside in order to measure the passage of the hours. Now, when people speak of the "sands of time passing", you should have a better idea of what is behind that statement.