That's done with ntpd. This page
probably gives you more information than you need to know, but basically you run ntpd at startup and set up a config file so it knows what servers to use. If you're in North America, "north-america.pool.ntp.org" will select an appropriate random server (unless you need extreme accuracy in which case that might not be sufficient). If you just want to update once, you can just do "ntpdate north-america.pool.ntp.org" (although that command is now deprecated in favor of ntpd).