In orde to make your PHP (symfony and Zend Framework too) application aware of your time zone, set the date.timezone php configuration variable in a .htaccess file, like this:
.htaccess contents:
php_value date.timezone America/New_York
From Plesk, you can create and edit this file from the File Manager located inside your domain section.
You can verify the change by creating a PHP script with the following content:
<?php echo date('Y-m-d H:i:s e' );
Which will print the time and the time zone.
