I've been noticing these messages lately on some of my Drupal 6 sites. These popped up any time I saved any content.
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/Chicago' for 'CDT/-5.0/DST' instead in /var/www/polycotassociates.com/modules/node/node.module on line 814.
Strangely, the fix was to put the following in the sites/default/settings.php file:
Update 2-6-13: I had to remove this line as I started getting the errors again.