Might also be that you haven't configured date.timezone in your php.ini.
You get this message because you get an exception either in the exception handler, or in the shutdown handler. Unfortunately PHP is crap at reporting these errors...
Normally it's a server setting, so it should be correct for all PHP applications running on the server. So it's best to fix it in php.ini. If you don't have access to that, the index.php is an alternative.