Windows PC freeware that synchronizes a PCs clock to an SNTP server.
Edit by : SNTP stands for Simple Network time Protocol.
Computers can use SNTP to get accurate time.

MasterSyncPC, a small program running on Microsoft Windows on a PC, uses SNTP to make that PCs clock accurate by synchronizing it to a reference. That reference is called an SNTP server, or time server, which sends time data over a network. That network can be the Internet or the Ethernet LAN that your PC is on.

An SNTP server is a software program running on a microprocessor--like the one in a PC or in a special purpose network device like the ones that Masterclock makes. This server can respond to requests for the time or send out the time periodically without being asked, or both. The time is a packet of data holding timestamps referenced to UTC time, which is the time at Greenwich, England. MasterSyncPC corrects your PCs clock by using those timestamps.

Making a request for time is called polling the server. When the server sends time packets without being asked it is said to be broadcasting. If you put a time server on your own Ethernet LAN then MasterSyncPC can poll it or listen for its broadcasts, but an Internet time server will only be polled because the broadcasting of SNTP time is not done on the Internet.

Time servers on the Internet are of 2 levels or strata. Those in Level 1 are extremely accurate, and have the task of serving time to those in Level 2 that in turn respond to requests from the public. Only Level 2 servers are appropriate for use by MasterSyncPC, as those in Level 1 should not be queried by end users, and are likely to be too busy.

Even a level 2 server may be very busy. This is noted on the website of U.S. servers http://tf.nist.gov/tf-cgi/servers.cgi
Note that website recommends some servers for new users.

NIST servers are available to anyone on the Internet from anywhere on EARTH, but there are many servers that restrict their responses to requests from within a service area. you can free download MasterSyncPC 2.0 now.