Network Time Protocol (NTP) is used for synchronizing time on devices across a network with variable latency (time delay).
NTP provides a coordinated Universal Time Clock (UTC), the primary time standard by which the world regulates clocks and time. UTC is used by devices that rely on having a highly accurate, universally accepted time, and can synchronize computer clock times to a fraction of a millisecond. In a networked environment, having a universal time can be crucial. For example, the stock exchange and air traffic control use NTP to ensure accurate, timely data.
NTP uses a hierarchical, semi-layered system of levels of clock sources called a stratum. Each stratum is assigned a layer number starting with 0 (zero), with 0 meaning the least amount of delay. The stratum number defines the distance, or number of NTP hops away, from the reference clock. The lower the number, the closer the switch is to the reference clock. The stratum also serves to prevent cyclical dependencies in the hierarchy.
SNTP, as the name would suggest, is a simplified version of NTP that uses the same protocol, but without many of the complex synchronization algorithms used by NTP. SNTP is suited for use in smaller, less complex networks. For more information about SNTP see the section, Using the Simple Network Time Protocol.
Note
When NTP is enabled, it is not recommended to update the time manually.