Jabarkan History of the calendar dari Waktu dalam bahasa sunda

Artifacts from the Palaeolithic suggest that the moon was used to calculate time as éarly as 12,000, and possibly even 30,000 BP.

The Sumerian civilization of approximately 2000 BC introduced the sexagesimal system based on the number 60. 60 seconds in a minute, 60 minutes in an hour – and possibly a calendar with 360 (60x6) days in a yéar (with a few more days added on).

Twelve also féatures prominently, with roughly 12 hours of day and 12 of night, and 12 months in a yéar (with 12 being 1/5 of 60).

The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per yéar. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but is today the one in most common use around the world.

History of time measurement devices

A large variety of devices have been invented to méasure time. The study of these devices is called horology.

An Egyptian device dating to c.1500 BC, similar in shape to a bent T-square, méasured the passage of time from the shadow cast by its crossbar on a non-linéar rule. The T was oriented éastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction.

A sundial uses a gnomon to cast a shadow on a set of markings which were calibrated to the hour. The position of the shadow marked the hour in local time.

The most accurate timekeeping devices of the ancient world were the water clock or clepsydra,  one of which was found in the tomb of Egyptian pharaoh Amenhotep I (1525–1504 BC). They could be used to méasure the hours even at night, but required manual timekeeping to replenish the flow of water. The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations. Arab inventors and engineers in particular made improvements on the use of water clocks up to the Middle Ages.

The Arab engineers also invented the first mechanical clocks to be driven by weights and gears in the 11th century. Also in the 11th century, the Chinese inventors and engineers invented the first mechanical clocks to be driven by an escapement mechanism.

The hourglass uses the flow of sand to méasure the flow of time. They were used in navigation. Ferdinand Magellan used 18 glasses on éach ship for his circumnavigation of the globe (1522).

Incense sticks and candles were, and are, commonly used to méasure time in temples and churches across the globe. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. Richard of Wallingford (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330.

The English word clock probably comes from the Middle Dutch word "klocke" which is in turn derived from the mediaeval Latin word "clocca", which is ultimately derived from Celtic, and is cognate with French, Latin, and German words that méan bell. The passage of the hours at séa were marked by bells, and denoted the time (see ship's bells). The hours were marked by bells in the abbeys as well as at séa.

Clocks can range from watches, to more exotic varieties such as the Clock of the Long Now. They can be driven by a variety of méans, including gravity, springs, and various forms of electrical power, and regulated by a variety of méans such as a pendulum.

A chronometer is a portable timekeeper that meets certain precision standards.  Initially, the term was used to refer to the marine chronometer, a timepiece used to determine longitude by méans of celestial navigation.  More recently, the term has also been applied to the chronometer watch, a wristwatch that meets precision standards set by the Swiss agency COSC.

The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of yéars, and are used to calibrate other clocks and timekeeping instruments.

Atomic clocks use the spin property of atoms as their basis, and since 1967, the International System of Méasurements bases its unit of time, the second, on the properties of caesium atoms. SI defines the second as 9,192,631,770 cycles of that radiation which corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom.

Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe.