When running cron.php from CLI you might get a warning like "It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'Europe/Berlin' for 'CEST/2.0/DST' instead ..."
Best solution is to set the time zone in the CLI version of php.ini, suggested in http://drupal.org/node/615438#comment-2195208