History of Timekeeping:

For thousands of years, devices have been used to measure and keep track of time. The current sexagesimal system of time measurement dates to approximately 2000 BC, in Sumer. The Ancient Egyptians divided the day into two 12-hour periods, and used large obelisks to track the movement of the Sun. They also developed water clocks, which were probably first used in the Precinct of Amun-Re, and later outside Egypt as well; they were employed frequently by the Ancient Greeks, who called them clepsydrae. The Shang Dynasty is believed to have used the outflow water clock around the same time; the clocks were introduced from Mesopotamia, possibly as early as 2000 BC. Other ancient timekeeping devices include the candle clock, used in China, Japan, England and Iraq; the timestick, widely used in India and Tibet, as well as some parts of Europe; and the hourglass, which functioned similarly to a water clock.
The earliest clocks relied on shadows cast by the sun, so they were not useful in cloudy weather or at night, and required recalibration as the seasons changed if the gnomon was not aligned with the earth's axis. The first clock with an escapement, which transferred rotational energy into intermittent motions, appeared in China in the 8th century, and Arabic engineers invented water clocks driven by gears and weights in the 11th century. Mechanical clocks employing the verge escapement mechanism were invented in Europe at the turn of the 14th century, and became the standard timekeeping device until the spring-powered clock and pocket watch in the 16th century, followed by the pendulum clock in the 18th century. During the 20th century, quartz oscillators were invented, followed by atomic clocks. Although first used in laboratories, quartz oscillators were both easy to produce and accurate, leading to their use in wristwatches. Atomic clocks are far more accurate than any previous timekeeping device, and are used to calibrate other clocks and to calculate the proper time on Earth; a standardized civil system, Coordinated Universal Time, is based on atomic time.

EARLY TIMEKEEPING DEVICES
Many ancient civilizations observed astronomical bodies, often the Sun and Moon, to determine times, dates, and seasons. Methods of sexagesimal timekeeping, now common in Western society, first originated nearly 4,000 years ago in Mesopotamia and Egypt; a similar system was developed later in Mesoamerica. 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.

CLOCKMAKERS
The first professional clockmakers came from the guilds of locksmiths and jewellers. Clockmaking developed from a specialized craft into a mass production industry over many years. Paris and Blois were the early centers of clockmaking in France. French clockmakers such as Julien Le Roy, clockmaker of Versailles, were leaders in case design and ornamental clocks. Le Roy belonged to the fifth generation of a family of clockmakers, and was described by his contemporaries as "the most skillful clockmaker in France, possibly in Europe". He invented a special repeating mechanism which improved the precision of clocks and watches, a face that could be opened to view the inside clockwork, and made or supervised over 3,500 watches. The competition and scientific rivalry resulting from his discoveries further encouraged researchers to seek new methods of measuring time more accurately.
Between 1794 and 1795, in the aftermath of the French Revolution, the French government briefly mandated decimal clocks, with a day divided into 10 hours of 100 minutes each.The astronomer and mathematician Pierre-Simon Laplace, among other individuals, modified the dial of his pocket watch to decimal time. A clock in the Palais des Tuileries kept decimal time as late as 1801, but the cost of replacing all the nation's clocks prevented decimal clocks from becoming widespread. Because decimalized clocks only helped astronomers rather than ordinary citizens, it was one of the most unpopular changes associated with the metric system, and it was abandoned.
In Germany, Nuremberg and Augsburg were the early clockmaking centers, and the Black Forest came to specialize in wooden cuckoo clocks.The English became the predominant clockmakers of the 17th and 18th centuries. Switzerland established itself as a clockmaking center following the influx of Huguenot craftsmen, and in the 19th century, the Swiss industry "gained worldwide supremacy in high-quality machine-made watches". The leading firm of the day was Patek Philippe, founded by Antoni Patek of Warsaw and Adrien Philippe of Berne.

MARINE CHRONOMETERS
Marine chronometers are clocks used at sea as time standards, to determine longitude by celestial navigation.They were first developed by Yorkshire carpenter John Harrison, who won the British government's Longitude Prize in 1759. Marine chronometers keep the time of a fixed location—usually Greenwich Mean Time—allowing seafarers to determine longitude by comparing the local high noon to the clock.

CHRONOMETERS
A modern quartz watch and chronograph
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 means of celestial navigation. More recently, the term has also been applied to the chronometer watch, a wristwatch that meets certain precision standards set by the Swiss agency COSC. Over 1,000,000 "Officially Certified Chronometer" certificates, mostly for mechanical wrist-chronometers—wristwatches—with sprung balance oscillators, are delivered each year, after passing the COSC's most severe tests, and being singly identified by an officially recorded individual serial number. According to COSC, a chronometer is a high-precision watch, capable of displaying the seconds and housing a movement that has been tested over several days, in different positions, and at different temperatures, by an official, neutral body. To meet this requirement, each movement is individually tested for several consecutive days, in five positions, and at three temperatures. Any watch with the designation chronometer has a certified movement.

QUARTZ OSCILLATORS
Internal construction of a modern high performance HC-49 package quartz crystal.
The piezoelectric properties of crystalline quartz were discovered by Jacques and Pierre Curie in 1880. The first quartz crystal oscillator was built by Walter G. Cady in 1921, and in 1927 the first quartz clock was built by Warren Marrison and J. W. Horton at Bell Telephone Laboratories in Canada. The following decades saw the development of quartz clocks as precision time measurement devices in laboratory settings—the bulky and delicate counting electronics, built with vacuum tubes, limited their practical use elsewhere. In 1932, a quartz clock able to measure small weekly variations in the rotation rate of the Earth was developed.[138] The National Bureau of Standards (now NIST) based the time standard of the United States on quartz clocks from late 1929 until the 1960s, when it changed to atomic clocks.[139] In 1969, Seiko produced the world's first quartz wristwatch, the Astron.[140] Their inherent accuracy and low cost of production has resulted in the subsequent proliferation of quartz clocks and watches.[91]