i actually had the same problem.
after manually creating the table tags in mysql, selfoss stopped with the same timezone message.

changing the line
date_default_timezone_set($val);
to
date_default_timezone_set("Europe/Berlin");
removed the message, but selfoss stopped with a blank page.

however, going back to the original code and adding the line
date.timezone = "Europe/Berlin";
to the php.ini did it!

so selfoss seems to work for me for now