Sometimes six months after you write a post you decide to update it with some new information. This simple snippet will add a modified date to your posts in the event that you make a change sometime after the initial publication date.

Posted on <?php the_time('F jS, Y') ?>
          $u_time = get_the_time('U');
          $u_modified_time = get_the_modified_time('U');

      if ($u_modified_time != $u_time) {
                echo "and last modified on ";
                the_modified_time('F jS, Y');
                echo ". ";