Recently we came across this red/pink error message on an old(ish) Drupal 6 site that is running on shared hosting:
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()
... blah blah blah ...
It seems that this PHP warning is new to PHP 5.3 so we reckoned that the shared hosting must have upgraded their PHP to 5.3, anyway it turned out that adding the following line to the site’s php.ini file got rid of the red error message:
date.timezone = 'Europe/Dublin'
This explicitly sets the time zone to ‘Europe/Dublin’