When you synchronize your authoritative time server with an external Network Time Protocol (NTP) time server, Windows translates the time to your time zone and makes adjustments for Daylight Saving Time. In addition, the OS fine-tunes the time to account for any delay that occurred while it received the information over the Internet.
Time servers, which are available around the world, are maintained in a hierarchy. Primary (stratum 1) servers are the most accurate, but secondary (stratum 2) servers are generally either synchronized perfectly with stratum 1 servers or are only slightly off the clock ticks of the stratum 1 servers. The difference, a matter of a few nanoseconds, certainly won't create a problem for the network features that use Windows time services. Because stratum 1 time servers are very busy and can time out, you should select stratum 2 servers as your external NTP servers.
For a list of stratum 1 servers, visit http://www.eecis.udel.edu/~mills/ntp/clock1.htm. For a list of stratum 2 servers, visit http://www.eecis.udel.edu/~mills/ntp/clock2.htm. Each list contains the NTP time servers available for public access, including any restrictions on their use. Each entry gives the country code, state (US only), host name, Internet address, and geographic information.
In the United States, the US Naval Observatory (USNO) maintains two popular external time servers: ntp2.usno.navy.mil (188.8.131.52) and tock.usno.navy.mil (184.108.40.206). In Canada, popular servers include clyde.concordia.ca (220.127.116.11), manitou.cs.concordia.ca (18.104.22.168), and xntp.ece.concordia.ca (22.214.171.124).