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') ?> <?php $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 ". "; } ?>