The NTP time server has revolutionised the synchronisation of computer networks over the last twenty years. NTP (Network Time Protocol) is the software that is responsible for distributing time from the time server to the entire network, adjusting machines for drift and assuring accuracy.
NTP can reliable maintain system clocks to within a few millimetres of UTC (Coordinated Universal Time) or whatever timescale it is fed with.
However NTP can only be as reliable as the time source that it receives and as UTC is the global civil timescale it depends on where the UTC source comes from.
National time and frequency transmissions from physics labs like NIST in the USA or NPL in the UK are extremely reliable sources of UTC and NTP time servers are designed specifically for them. However, the time signals are not guaranteed, they can drop off throughout the day and are susceptible to interference; they are also regularly turned of for maintenance.
For most applications a few hours of your network relying on crystal oscillators will probably not cause too much problems in synchronisation. However, GPS (Global Positioning System) is far more reliable source for UTC time in that a GPS satellite is always overhead. They do require a line-of-sight reception which means an antenna has to go on the roof or outside an open window.
For applications where accuracy and reliability are essential the safest solution is to invest in a dual system NTP time server, these device can receive both the radio transmissions such as MSF, DCF-77 or WWVB and the GPS signal.
On a dual system NTP server, NTP will take both time sources and to synchronise a network to ensuring increased accuracy and reliability.