To set the system clock, use the date command.
For Redhat Systems, use the following format:
date --set="Mmm DD HH:MM:SS YYYY"
Example:
date --set="Oct 20 15:52:29 2004"
For FreeBSD systems, use the following format:
date YYMMDDHHMM
Example:
date 0410201544
Will set the date/time to 2004, October 20th, 15:44 (3:44pm).
For any operating system, if you have the rdate program, you can simply type:
rdate -s time-a.nist.gov
to sync your server with the atomic clock in Boulder, Colorado. Note that if the value set by rdate isn't correct, then you likely have a wrong timezone specified. Commands like system-config-date or redhat-config-date can set it up for you. Else you'd need to create a symbolic link from one of the timezones in /usr/shared/zoneinfo to /etc/localtime, eg:
mv /etc/localtime /etc/localtime.moved ln -s /usr/share/zoneinfo/Canada/Mountain /etc/localtime