Change the post date from a meta box

A Note: Actions and Filters are not really interchangeable: Filters typically must return the passed data or it’s going to break something.

Building off of what m0r7if3r said, wp_insert_post_data is a filter, so you should be modifying the post’s $data and returning it at the end of the function.

(Alternatively, you could global the variables you need to alter during an appropriate action, then change it.)

for a tutorial, first read: http://core.trac.wordpress.org/browser/tags/3.3.1/wp-includes/post.php#L2566

Then consider the following (I haven’t tested this but it’s based on your code):

function cfc_reset_postdate( $data ) {
    $date = get_post_meta( get_the_ID(), 'cfc_date', true );
    $date = DateTime::createFromFormat('D - M j, Y', $date);
    $date = $date->format('Y-m-d');

    $data['post_date'] = $date;
    return $data;
}

add_filter( 'wp_insert_post_data', 'cfc_reset_postdate');