Timekeeping developed in early recorded history, with different societies using their natural surroundings and devices they created to help keep track of time
. The basic units we use to keep time have not evolved greatly, showing their ancient historical roots. The keeping of time was important to early agriculture, particularly in irrigation where timing access to water was important. Increasingly, timekeeping was not just conducted by the select few by much of society.
==Early Timekeeping Devices==
Some of the earliest timekeeping devices involved the use of water, or what were called water clocks or <i>clepsydrae</i> as called by the Greeks. This involved either using outflow of water or inflow. Effectively, as water drained out or filled in, the rate was seen as relatively constant based on marking on containers would then tell how much time had passed as the water drained or filled in. Such devices were known to have existed in many of the earliest complex society in Eurasia, including in Egypt, Mesopotamia, the Indus, China, and likely other regions, including ancient Greece and Rome. In China during the Han dynasty, water clocks were critical for astronomical observations and became an increasingly complex set of tanks that also had secondary tanks that would provide additional water to keep water moving to measure time as pressure changes could mean that the rate of which water moved changed, thus potentially invalidating any time measure.<ref>For more on some of the earliest time devices, see: Jespersen, J., Fitz-Randolph, J., Robb, J., & Miner, D. (1999). <i>From sundials to atomic clocks: understanding time and frequency (1999 edition)</i>. Washington, D.C: U.S. Dept. of Commerce, Technology Administration, National Institute of Standards and Technology. </ref>
Other early devices include candle clocks, which were probably only used indoors. These were also throughout much of Eurasia from the Bronze Age and throughout the Medieval period. Candle clocks were simple, where the rate of burning would be constant and the candle would be pre-measured so that different levels in which the candle melted as it burned would help tell time. The sundial is likely another ancient device. Both the Babylonians and Egyptians used the sundial at least by the 2nd millennium BCE if not earlier (Figure 1).
The sexagesima system we use in our own clocks today was already used and invented by the Sumerians and Babylonians, which was also likely used for initial sundials. The oldest sundial appears to date from around 1500 BCE, but sundials are likely much older.<ref>For more on early units and systems of timekeeping, see: Wonning, P. (2018). A History of Time: A Chronicle of Calendars, Clocks and Time Zones. </ref>
Greek philosopher Philo in the 3rd century BC and in China, under development by the mathematician Yi Xing by the 7th century, escapement clocks were built, which use a mechanical device that releases a gear which then allows the rate of release to be used as a timekeeping element. This is similar conceptually to our mechanical clocks and such clocks would begin to signal the increasing realization that mechanical timeclocks would be the only an easier way to keep time more consistent. Initially, devices would still depend on water or water pressure to help push levers and mechanical devices to tick at given intervals.<ref>For more on how Philo and Chinese developments helped evolve timekeeping, see: Usher, A. P. ( 1988). <i>A history of mechanical inventions (Rev. ed)</i>. New York: Dover. </ref>
[[File:Sundial-egypt. jpg|thumb|Figure 1. The oldest sundial found, altough these objects are probably more ancient, derives from Egypt from the 2nd millennium BCE. ]]
By the Medieval period, clocks continued to use some of the older methods, such as sundials and water clocks, while also incorporating newer methods that included escapements. Other inventions by this time also included the hourglass, which was first known to have been used by the early Medieval period in the 8-9th centuries in Germany. The origins of the hourglass, however, could be much older. The key inventions were more sophisticated tower clocks that used more advanced water clocks and hydraulics, which would wind and use torque in mechanical component to tick at given intervals, such as every hour. Early examples include the Great Mosque in Damascus and the Mustansiriya school in Baghdad. One advance came in the early 13th century under Al-Jazari, who was an Islamic scholar who perhaps created one of the first fully automated clocks and even created a clock that would have programmable automata musicians that played for guests and could be reprogrammed. Some have described this as the earliest form of computer programming. Nevertheless, by the 13th century, mechanical clocks were becoming more common, although often still powered by water. Town squares throughout Europe were also putting up tower clocks as a way for townspeople to keep track of time and to be informed of important events such as time to prayers. Both in Christianity and Islam, time became an important element in prayers and religious activities, driving much research in improving clocks and making them more audible and visible as well. This is the case in Europe with bells being now attached to the mechanical devices of clocks so that sound could be used to inform on important time changes, often the change in the hour.
In the 10th century, monks throughout Europe, who were
particularly concerned with time to regulate their daily schedules and prayers, were experimenting with mechanical clocks more regularly. This included creating mechanical wheels and gears that would tick using the principals developed in escapements. The oldest functioning clock might be the clock in Salsbury Cathedral, which dates to the late 14th century (Figure 2). This uses a series of mechanical wheels that used a series of wheels and was set to strike at given intervals, probably every hour . Increasingly, such devices that used gears and wheels to tick at set intervals were used. Throughout the 14th century, and probably as early as the 11th century, mechanical clocks were becoming more regular features in cities, churches, and cathedrals in particular.
[[File: Salisbury Cathedral clock-1060x450. jpg|thumb|Figure 2. Perhaps the oldest known working clock is the clock in Salsbury cathedral, which dates to the late 14th century. ]]
==Mechanical Revolution and Watches==
By the 14th century, mechanical clocks in Europe to become more common. Clocks, similar to how they were used in China and Middle East, could also be used in conjunction with astronomical events, such as in predicting positioning of the moon, sun, and other objects. These astronomical clocks tried to keep of time and better understand how the amount of light might change on a daily basis. Mechanical clocks now began to have, by the 14th-15th centuries, more sophisticated appearances, including the use of weights. Pendulums were studied by Galileo but first utilised by Christiaan Huygens in 1656, where swinging of the pendulum would help the escapement to be put in place. Early clocks were not as accurate as sundials, where they often had to be rewound. Equation clocks, which tried to adjust for variation in ticking mechanical clocks, were also created to adjust for the variation in time among mechanical clocks. These clocks would try to measure solar time throughout the day rather than only during daylight hours. These equation clocks would try to self-adjust as ticking mechanical clocks often slowed down, where the equation clocks could then be used to reset the mechanical clocks.
By the 16th century, clocks were becoming more common. Now, individuals wanted to be able to better keep time as they moved about their day rather than have to look up or hear the clocktower ring out. Small versions of mechanical clocks
were made that used miniature wheels and gears to tick and keep time, which were the first watches. Watches were often carried inside a coat pocket because the elements could foul or corrode the devices in the watch. Some wristwatches were worn, mostly by women, but these were often avoided due to the harm they might be exposed to.
Wristwatches developed to be more common only by the late 19th century. During the Boer War, soldiers began to wear wrist watches in order to better coordinate troop movements using time. Having a watch in a coat would potentially expose the soldier as they reached for their pockets. Although wristwatches were available as fashion items, often as part of bracelets for ladies, they now became more part of men fashion. By 1900, wristwatches became fashionable among men and began to compete with pocket watches. During the early days of aviation, wristwatches became critical as it was easier to tell time using one's wrist. Wilsdorf & Davis, a company in England, began to create specifically made wrist watches which had stronger frames and held better than other wristwatches, which were often not very different from pocket watches. The company would eventually become Rolex and they helped to popularize the idea of fashionable wristwatches among men. The widespread use of wristwatches in World War I also increased the market for wristwatches in the wider public.
The first digital clock was created by the Austrian Josef Pallweber in 1883, which used a rotating disk to display time. The Plato clocks similarly used small digital plates that would be displayed. These clocks were displayed
in the St. Louis World Fair in 1904. It took a few more decades for the invention of the digital alarm clock, which appeared in 1956 after being invented by D. E. Protzmann. Digital clocks and wristwatches only become more common from around 1970, as new patents enabled better displays and LED technologies.