this error means that the date.timezone is not set in the servers php.ini file:
datetime.configuration.php#ini.date.timezone
you will need to contact the host to have them correct the issue.
alternately, you can add the following code at line 1 of the form page to set the timezone:
<?php date_default_timezone_set('America/Chicago'); ?>
The list of valid timezones is here...
http://www.php.net/manual/en/timezones.php