A network time server will synchronise all machines and devices on a network to UTC time (coordinated universal time). They are essential pieces of hardware in modern computer networks, without which, time sensitive transactions would be impossible.
Most network time servers will use NTP (Network Time Protocol) to distribute a single UTC time source (Coordinated Universal Time) around a network. The way a network time sever synchronises devices is by allowing them to ask it the time (poll) if the device is slow or fast compared to UTC then NTP will advance or retreat the system clock.
You would think that on larger networks with hundreds of machines all requesting timing information from a single network time server will cause congestion of the network and the time server. However, NTP is hierarchical, the distance from the time source is split into strata, the closer to the time source the lower the stratum level. So for example a stratum 0 device is an atomic clock while a dedicated network time server that receives the time from an atomic clock is a stratum 1 device while a computer that receives the time from a network time server is a stratum 2 device.
Fortunately stratum 2 devices can also be used to distribute time. Devices that receive time from a stratum 2 device become stratum 3 stratum device. These devices too can be used to distribute time to lower stratum levels, although the accuracy of the devices will lessen the further away from the original time source.
Arranging a network into stratum levels means that all devices can be synchronised using just the one network time server.
