That is set in the php.ini. Information on how to do this is located at:
http://php.net/manual/en/datetime.configuration.php(http://php.net/manual/en/datetime.configuration.php)
Basically you need to add the following to your php.ini
date.timezone = "Your Time Zone Area"
A complete list of timezones support is available here:
http://www.php.net/manual/en/timezones.php(http://www.php.net/manual/en/timezones.php)
