Pernahkah menjumpai script buatan Anda sendiri atau hasil copy paste tampilan Warning seperti di bawah ini sewaktu dijalankan di browser?

Warning: date_default_timezone_get() [function.date-default-timezone-get]: 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 ...

Apa sih penyebabnya? dan bagaimana solusi untuk menghilangkan Warning tersebut?