To set the correct timezone edit conf/local.protected.php1) and add the line:
putenv("TZ=GMT");
If you use php5:
date_default_timezone_set("GMT");
Note: Replace GMT with your specific timezone.
You can find a list of timezones here: http://www.theprojects.org/dev/zone.txt
conf/local.php, your changes will be lost when you configure via the GUI