Erreur php

Bonjour,

j'ai l'erreur suivante :

Warning: 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 'America/New_York' for 'EDT/-4.0/DST' instead in /var/www/vhosts/monsite.com/httpdocs/ajax/stream.php on line 351

et voila a quoi correspond la ligne 351 :

$html.='<span class="notopmargin small-italic" style="font-weight:normal">'.$misc->ago(strtotime($event['event_date']),$lang).'</span>';

Merci de m'aider à corriger ce problème.