<P> Atomic clocks are far more accurate than any previous timekeeping device, and are used to calibrate other clocks and to calculate the International Atomic Time; a standardized civil system, Coordinated Universal Time, is based on atomic time . </P> <P> Many ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine times, dates, and seasons . The first calendars may have been created during the last glacial period, by hunter - gatherers who employed tools such as sticks and bones to track the phases of the moon or the seasons . Stone circles, such as England's Stonehenge, were built in various parts of the world, especially in Prehistoric Europe, and are thought to have been used to time and predict seasonal and annual events such as equinoxes or solstices . As those megalithic civilizations left no recorded history, little is known of their calendars or timekeeping methods . Methods of sexagesimal timekeeping, now common in both Western and Eastern societies, are first attested nearly 4,000 years ago in Mesopotamia and Egypt . Mesoamericans similarly modified their usual vigesimal counting system when dealing with calendars to produce a 360 - day year . </P> <P> The oldest known sundial is from Egypt; it dates back to around 1500 BC (19th Dynasty), and was discovered in the Valley of the Kings in 2013 . Sundials have their origin in shadow clocks, which were the first devices used for measuring the parts of a day . Ancient Egyptian obelisks, constructed about 3500 BC, are also among the earliest shadow clocks . </P> <P> Egyptian shadow clocks divided daytime into 12 parts with each part further divided into more precise parts . One type of shadow clock consisted of a long stem with five variable marks and an elevated crossbar which cast a shadow over those marks . It was positioned eastward in the morning, and was turned west at noon . Obelisks functioned in much the same manner: the shadow cast on the markers around it allowed the Egyptians to calculate the time . The obelisk also indicated whether it was morning or afternoon, as well as the summer and winter solstices . A third shadow clock, developed c. 1500 BC, was similar in shape to a bent T - square . It measured the passage of time by the shadow cast by its crossbar on a non-linear rule . The T was oriented eastward in the mornings, and turned around at noon, so that it could cast its shadow in the opposite direction . </P>

When did humans start keeping track of time