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function update_post_title_from_customfield( $post_id ) {

  $saved_post = array();
  $saved_post['ID'] = $post_id;

  $field = get_field('date_date_day');

  if (get_post_type() == 'date' && $field) {
    $saved_post['post_title'] = $field;
  } 

  wp_update_post( $saved_post );
}
   
add_action('acf/save_post', 'update_post_title_from_customfield', 20);