It is a question that has perplexed philosophers and scientists since the dawn of man, ‘what exactly is time?’ and it has only been in our recent history that we have started to discover answers, thanks to Einstein and his work on special and general relativity.
We now know time is not the abstract concept we first thought it was, we also know it is not constant and is relative to different observers throughout the universe with the speed of light being the only constant in the universe.
In other words if the speed of light has to be the same for everybody then someone travelling at close to such a speed would find time slow down.
Fortunately as all humans live within the boundaries of the planet Earth it means the passing of time is very similar for us all (or so minutely different as to be impossible to measure). However,
As humans have progressed, telling the time with ever increasing accuracy has become more and more important. Historically, knowing the time was not so imperative. People needed to know the correct day to plant crops or when sunrise and sunset happened but accuracy was not a preoccupation.
Seafaring, aviation and now space travel mean that humans have sought more and more accurate ways of keeping time.
In the 1950’s atomic clocks were developed which were so accurate it was discovered that the revolution of the Earth, something we had based our timescale on for centuries, was no where near as accurate as these new clocks.
Fortunately a global time scale, UTC (Coordinated Universal Time), has been developed and is based on the time told by atomic clocks. This allows systems all over the world to be synchronised to the exact same time.
Computer networks use the NTP protocol (Network Time Protocol) to receive a UTC timing reference and synchronise all machines on a network to that time.
NTP servers can receive a time reference over the Internet (although not very secure) from a national radio transmission (as long as the receiver is within range of a suitable transmission) or from the GPS network (via a rooftop GPS antenna).