NTP (Network Time Protocol) and the associated software is a science in itself. The round-trip latency is used in the time calculations, as is the averaging of multiple servers to find correct servers and outliers. With good hardware, microsecond to nanosecond accuracy is possible.(The network time protocol project) has a lot of information, including scientific papers.
Koos van den Hout
Camp Wireless, wireless Internet access at campsites| Koos van den Hout| keyid DSS/1024 0xF0D7C263 or RSA/1024 0xCA845CB5| Fax +31-30-2817051 [TELECOM Digest Editor's Note: Thanks for mentioning this site. I went over earlier today and selected from the 'us.pool.ntp.org' numbers for my computer clock settings. PAT]