It is not safe to rely on the system's timezone settings.

PHP Warning: date(): 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. We selected 'Asia/Tokyo' for 'JST/9.0/no DST' instead in /opt/hoge/lib/jikken.inc on line 87

PHPバージョンアップの際に発生するタイムゾーンエラーの対処方法 - kngy．net
タイムゾーンちゃんと設定しなさいというwarningらしい