Error Time pada Joomla

Error yang nenampilkan pesan berikut :

Warning: mktime() [function.mktime]: 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 'Europe/London' for 'BST/1.0/DST' instead in /home/    /public_html/libraries/joomla/utilities/date.php on line 117

Bisa kita taruh kode berikut dibagian atas libraries/joomla/utilities/date.php

<?php date_default_timezone_set('America/New_York'); ?>