I thought you meant a fatal error, I had to double check it. It's an E_WARNING. date.timezone will actually fall back to UTC if you *don't* set it, this is important because that can cause unepected results.
"PHP will no longer attempt to guess the timezone, and will instead fall back to "UTC" and issue a E_WARNING." http://php.net/ChangeLog-5.php#5.4.0