The latest releases (5.3.x) of PHP require you to set the timezone and will otherwise throw an error if not set. This can be done one of two ways:
Via php.ini file (takes care of entire site)
date.timezone = "America/New_York"
NOTE: For sites on media temple, create a new php.ini file in /home/xxxxx/etc/ and enter the above.
The second way is via the .htaccess file on a directory-by-directory basis:
php_value date.timezone "America/New_York"