Telling the time may seem a uncomplicated affair these days with the whole of devices that display the time to us and with the startling accuracy of devices such as atomic clocks and network time servers it is quite easy to see how chronology has been taken for granted.
The nanosecond accuracy that powers technologies such as the Gps system, air traffic control and Ntp server systems (Network Time Protocol) is a long way from the first time pieces that were invented and were powered by the movement of the sun over the heavens.
Sun dials were easily the first real clocks but they obviously did have their downsides - such as not working at night or in cloudy weather, however, being able to tell the time fairly accurately was a unblemished innovation to civilisation and helped for more structured societies.
However, relying on celestial bodies to keep track of time as we have done for thousands of years, would not prove to be a dependable basis for measuring time as was discovered by the invention of the atomic clock.
Before atomic clocks, electronic clocks provided the top level of accuracy. These were invented at the turn of the last century and while they were many times more dependable than mechanical clocks they still drifted and would lose a second or two every week.
Electronic clocks worked by using the oscillations (vibrations under energy) of crystals such as quartz, however, atomic clocks use the resonance of personel atoms such as caesium which is such a high whole of vibrations per second it makes the incredibly spoton (modern atomic clocks do not drift by even a second every 100 million years).
Once this type of time telling accuracy was discovered it became apparent that our tradition of using the rotation of the earth as a means of telling time was not as spoton as these atomic clocks. Thanks to their accuracy it was soon discovered the Earth's rotation was not spoton and would slow and speed up (by exiguous amounts) each day. To compensate for this the world's global timescale Utc (Coordinated Universal Time) has supplementary seconds added to it once or twice a year (Leap seconds).
Atomic clocks contribute the basis of Utc which is used by thousands of Ntp servers to synchronise computer networks to.
Milestones in Chronology - From Crystals to Atoms