It is very similar to the other questions like What come first egg or hen? , men or women? and so on… People were dividing the day long before there were clocks. Dawn, noon and dusk are fairly obvious. To anyone who doesn't live very near the equator, it is also pretty clear that the length of days isn't constant. Sumerians and Egyptians both divided the day into either 12 or 24 periods, probably to match the 12 months of the year. It then took a while to figure out how to figure how the hours changed. In early systems, there were 12 daylight hours and 12 night time hours and the hours varied in length. (It helps that both those civilizations were fairly near the equator. The day-length ranges from about 10 to 14 hours. But lengths of time could be measured in various ways, e..g by burning lengths of rope. The history of measuring the time is much older than the history of the clock and watch. If we look into the history of the time measuring devices, these are 4000+ years old e.g. obelisks, water clocks, candle clock, timestick, hourglass, sundial. These all type of devices were mainly dependent on the natural resources like water, sand or sun light. Later on, few other mechanism has been made in 3rd century with water-powered escapement mechanism. Mercury-powered escapement mechanisms in the 10th century, and then gears and weights in the 11th century. Finally, foliot or balance wheel timekeeper which became the standard time keeping the device in the 14th century.