PHPで「Warning: date(): It is not safe to rely on the system's timezone settings.」のエラー

PHPでdate関数を使った際に、

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 the timezone 'UTC' for now, but please set date.timezone to select your timezone. in /hogehoge/hugahuga/hoge.php on line 42

というみたことの無いエラーが発生。

ファイルごとに以下のように記述するか、

date_default_timezone_set('Asia/Tokyo');

date.timezone = Asia/Tokyo