change /etc/localtime to UTC time?
You can set the time in System Preferences, and that time should be reflected back in /etc/localtime.

If you want to do it at the command line, use the systemsetup tool (which has to be run via sudo, fyi).

Here are the relevant commands from sudo systemsetup -help:

Usage: systemsetup -gettimezone
    Display current time zone.

Usage: systemsetup -settimezone &lt;timezone&gt;
    Set current time zone to &lt;timezone&gt;. Use "-listtimezones" to list time zones.

Usage: systemsetup -listtimezones
    List time zones supported by this machine.