The Simple Network Time Protocol (SNTP) is the most popular of the several time retrieval protocols. Server access requires using a SNTP client (clock). It is strongly recommended that SNTP servers never be queried more frequently than once every 15 seconds. Clients should utilize servers close to their geographic location whenever possible. Many servers have restricted access, but below are addresses to anonymous public SNTP servers.
[IANA Port assignment = 580]