Using save_post to replace the post’s title

I am using custom posts, and in these, I don’t a need for the title.

This causes WordPress to set the titles of my posts to “Auto Draft”.

I’d like to change the title’s value to something else, computed from other fields in my post.

How do I go about doing that using save_post or some other means?

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.
}