JOOMLA - Probleme de type Warning lié à timezone

Bonjour tout le monde!
je suis administrateur du site afrik11.com basé sur JOOMLA 1.5.26, et depuis quelque je suis confronté a un problème de time_zone.
En effet lorsque j'entre dans la gestion des articles de l'administration, voici les warning qui s'affichent:

 Warning: strtotime() [function.strtotime]: 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 'UTC' for 'GMT/0.0/no DST' instead in /home/afrikwry/public_html/libraries/joomla/utilities/date.php on line 56


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 'UTC' for 'GMT/0.0/no DST' instead in /home/afrikwry/public_html/libraries/joomla/utilities/date.php on line 117

Warning: strftime() [function.strftime]: 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 'UTC' for 'GMT/0.0/no DST' instead in /home/afrikwry/public_html/libraries/joomla/utilities/date.php on line 250

je tiens a préciser que cela se passe uniquement lorsque je veux éditer un nouvel article.
Alors je sollicite votre aide pour solutionner ce problème.

MERCI
Cordialement