Sometimes I will update a post and the following code snippets adds below the original date a message that says updated then the date I updated the post.

last-post-update.php

//* The following code was taken from: https://dl.dropboxusercontent.com/u/2503558/GenesisWP/code-snippets.html 
//*  and works only for child themes using the Genesis framework.
//*
//* Add last updated date to the post info in entry header
add_filter( 'genesis_post_info', 'sk_post_info_filter' );
function sk_post_info_filter($post_info) {

	if (get_the_modified_time() != get_the_time()) {
		$post_info = $post_info . '<br/>Last updated on: ' . the_modified_date('F j, Y', '', '', false);
	}
	return $post_info;

}