mktime関数によるエラー

mktime(): It is not safe to rely on the system's timezone settings. Please use the date.timezone setting, the TZ environment variable 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

これは、つまり、mktime()を使うときは、使う前にタイムゾーンを設定しないといかんのだよ、ということです。