It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier.
These have been warnings in earlier versions of PHP, but as of PHP 5.4, the
date.timezone ini setting must be set correctly, using the continent and place - for me that's "Europe/London", like this:
date.timezone = "Europe/London"
If you see these errors, don't panic, just add the line above to your php.ini.