Are there posts on your blog you regularly update rather than writing new separate entries such as tutorials for instance? Wouldn't it be nice if there was a way you could let your readers know when a post was updated other than adding an addendum to the bottom? Well there is.
WordPress outputs the date of a post by this tag,
Posted on <?php the_time('F jS, Y') ?>
You will find it any all of the template files that display posts such as index.php, single.php etc. Simply replace that bit of code with this,
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 ". "; } ?>
The result will look like this,

