Using php 5.x with Drupal 6.x I would often see this 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…
All it’s saying is that the timezone of your web application is undefined so the system is required to “guess.” I’ve seen some hacks out there but the right way to fix it is to edit your php.ini file like so
date.timezone = America/Los_Angeles
Use whatever your timezone is. List of supported timezones is here.