If you have NTP (Network Time Protocol) installed you can use ntpdate to set you date/time from a network time server on the Internet.
# ntpdate tick.usno.navy.mil
5 Apr 21:55:24 ntpdate: adjust time server 126.96.36.199 offset 0.005845 sec
ntpdate is best for only occasional use.
The more servers you query with ntpdate the better accuracy you will get. One is usually good enough for me. You can get lists of public ntp servers with a bit of googling.