I run these on my systems. One box is the local timeserver. It syncs against a number of outside servers. Accuracy is well under 1 second, [link|http://www.uwsg.iu.edu/hypermail/linux/kernel/9812.0/0306.html|kernel mail] from Peter Anvin suggests 200 - 50 microsecond accuracy (+/- 0.2-0.05 seconds). Relevant RFCs: 1305, 1059, and 1119.
Solaris is going to have something similar, not sure of NT.