Before mechanically driven or digital watches, timekeeping was more approximate but still surprisingly accurate. Well made hourglasses could be made that would reliably reproduce any desired period of time, from a minute to many days, with commendable accuracy. Some cultures used water instead, but the principle is the same: a substance passes through a restricted opening at a steady rate, meaning the amount of time that has passes is easily determined from the amount of that substance that has fallen. The hourglass then need only be flipped to begin again.
Of course, individual grains of sand are not particularly visible in a user interface, so shading is used to indicate the passage of sand, and usually it flips frequently to indicate time is passing.