Display ‘modified date’ in WordPress Twenty Fourteen

Look for the code that displays the post date. It should look something like the following:

Posted on: <?php the_time('l, F jS, Y') ?>

Now replace it with the following code (slightly modified from Ardamis’ post):
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 ". "; } ?>