By 
Time is a concept that has preoccupied the thinking of humans since the earliest civilizations. We may only have begun to understand time in the last century, thanks to the work of Albert Einstein, but measuring its pace has been an important part of society.
Historically, time has been measured using the Earth’s rotation and other astronomical cycles, such as the phases of the moon. The measurement of time, and thus the identification of key events during the day, month, or year, has been essential to the development of sophisticated, religious, and agricultural societies.
While calendars have been around for millennia, dating back to prehistoric monuments like Stone Henge, they are believed to indicate the winter and summer solstice and thus identify the longest and shortest days for planting crops, measure increments of time smaller has proven to be a technological challenge.
The first clocks were nothing more than stopwatches, using water, lighted candles and sand. These were successful in measuring a given period of time, but less useful in identifying how long it was before daylight or other important events would expire.
The sundial was the first real clock in which it was possible (on a sunny day) to divide the day into equal increments and thus provide a consistent measure of the day.
Mechanical clocks first appeared during the 14th century. While their technology was based on simple mechanics and they were less accurate than existing sundials, they provided a solution to the obvious drawbacks of the sundial (cloudy days).
Mechanical clocks really came into their own once the pendulum was developed in the 1600s, which ensured that existing mechanical clocks could provide greater precision. The development of pendulum-powered mechanical clocks skyrocketed over the next few centuries and they became increasingly accurate.
However, once it was discovered that certain crystals oscillate at exact speeds while under the influence of an electronic current, electronic watches soon took over as people realized that they provided much greater precision.
However, during the 1950s the atomic clock was developed, which used the oscillation of a single atom (usually cesium) that oscillated at an exact rate every second. Thanks to atomic clocks it was soon discovered that the traditional way of using the rotation of the Earth and other astronomical bodies as a basis for measuring time would soon cause problems as the Earth, it was discovered, would slow down or speed up its rotation due to the effects of the Moon’s gravity.
If nothing was done, then the time indicated by atomic clocks (International Atomic Time-TAI) would eventually lose synchronization with Earth and night would slowly turn to day.
A solution was found in UTC (Coordinated Universal Time) where leap seconds were added to compensate for this slowdown in the Earth’s rotation.
UTC and atomic clocks have made technologies such as satellites, global communication, and the Internet possible. Most computer networks are governed by an atomic clock through an NTP (Network Time Protocol) server. NTP servers receive the time signal from an atomic clock from a radio signal or from the US GPS (Global Positioning System) network. This allows computers around the world to synchronize to the same time scale ( UTC), which allows for time-sensitive transactions such as the stock market and Internet trading.