Using save_post to replace the post’s title

This simplest method would be to edit the data at the point it’s inserted, rather than updating it afterwards, using wp_insert_post_data instead of save_post. This works on creating a new post or updating an existing post without change. It also avoids the danger of creating an infinite loop by triggering update_post within save_post.

add_filter( 'wp_insert_post_data' , 'modify_post_title' , '99', 1 ); // Grabs the inserted post data so you can modify it.

function modify_post_title( $data )
{
  if($data['post_type'] == 'rating' && isset($_POST['rating_date'])) { // If the actual field name of the rating date is different, you'll have to update this.
    $date = date('l, d.m.Y', strtotime($_POST['rating_date']));
    $title="TV ratings for " . $date;
    $data['post_title'] =  $title ; //Updates the post title to your new title.
  }
  return $data; // Returns the modified data.
}