Your php.ini file should contain a setting for the time zone your server is in.

If for some reason this is not set (poorly configured host) or if you want to force the default time zone used by PHP in b2evolution to a different value, you can set it in the _advanced.php like this for example:

$date_default_timezone = 'Europe/Paris';

You can find the list of supported time zones here: http://www.php.net/manual/en/timezones.php

Setting a wrong value will display a PHP notice on your pages.