Most network time servers receive in some form UTC (Coordinated Universal Time). This global timescale is received either through a time and frequency transmission such as MSF (UK), DCF (DE) or WWVB (USA) or alternatively through the GPS network (strictly speaking GPS time is not UTC but NTP GPS server will convert the GPS signal into UTC).
UTC was developed after the inception of the atomic clock when a new incredibly accurate timescale called International Atomic Time was developed. At first it was thought this timescale could replace the much used GMT (Greenwich meantime) but it was soon discovered that the atomic clock and TAI was too accurate.
The problem was that the Earth’s rotation is minutely affected by the gravity of the Moon and would occasionally slow down and speed up.
These minute fraction of a second differences may not matter at first but if left unchecked eventually the day’s time would drift and midnight would eventually become midday (albeit in many centuries). Also many groups of people (mainly astronomers) complained that any drift would make their work impossible.
The solution was a new timescale that adjusted for the slowing of the Earth’s spin by adding Leap Seconds. The new time scale was called UTC (Coordinated Universal Time). Since its inception 33 Leap seconds have been added to UTC, normally one is added at the end of the year.
UTC is utilised by tens of thousands of network time servers for the process of synchonisation