If you’re running on PHP 5.3, you may have seen errors like this in your php.ini file:
Configuration variable date.timezone is not set, guessed timezone America/Chicago. Please set date.timezone="America/Chicago" in php.ini!
This is because a change in the how PHP determines what timezone you are using in PHP 5.3. Before PHP 5.3, PHP would rely on the host operating system’s timezone setting for this value. However, this isn’t considered reliable, so as of PHP 5.3 you need to specify it directly. You can do the easily by editing your php.ini file and adding the following line ( assuming your timezone in America/Chicago, see here for all the valid options ).
See this page in the PHP Documentation for more details.