这对我有用。将此放置在functions.php 文件：

function prefix_last_modified_date() { 
  $last_modified_date = null;

  $args = array(
    \'post_type\'  => \'post\', // could be page, or a CPT
    \'orderby\'    => \'modified\',
    \'post_count\' => 1,
  );

  $last_modified = new WP_Query( $args );

  if ( $last_modified->have_posts() ) {
    $last_modified->the_post();
    $last_modified_date = get_the_modified_date();
  }

  wp_reset_postdata(); 

  return $last_modified_date;
} 

echo prefix_last_modified_date();