Usually you'll want to have the time set automatically, and in that case, you'll want to set up
ntpd to automatically set the time for you.
The specifics differ slightly from distribution to distribution, but if you're running Ubuntu, for instance, there's a guide on setting up NTP on Ubuntu. Otherwise, just Google
ntpd <distribution-name>, and you'll probably find it.
If you want to set it manually, however, you can use
date --set="<date string>". Examples of this could be:
date --set="23 June 1988 10:00:00"