From GroupScript documentation
Time zone can be set in Backend->Settings->Localization.
If you use version 2.7 or older, the following steps must be taken:
Time zone can be set in system/application/config/config_local.php.
Look for line that reads
You have to change the value 'Europe/London' to one that is suitable for your time zone. It is possible, that your city is not in the list. In that case, please select the city closest to you which you know is in the same time zone. For the list of supported values, please see this entry in PHP documentation: http://php.net/manual/en/timezones.php