displaying last updated date & time on posts

Hi there.

On posts that I have updated after initially posting them, I want to display a "last updated" date in addition to the posting date. Initially I added this PHP via Code Snippets plugin, but it added the "last updated" text to pages as well as posts:

function wpb_last_updated_date( $content ) {
$u_time = get_the_time('U'); 
$u_modified_time = get_the_modified_time('U'); 
if ($u_modified_time >= $u_time + 86400) { 
$updated_date = get_the_modified_time('F jS, Y');
$updated_time = get_the_modified_time('h:i a'); 
$custom_content .= '<p class="last-updated">Last updated on '. $updated_date . ' at '. $updated_time .'</p>';  
} 
 
    $custom_content .= $content;
    return $custom_content;
}
add_filter( 'the_content', 'wpb_last_updated_date' );

Wp Beginner, at http://www.wpbeginner.com/wp-tutorials/display-the-last-updated-date-of-your-posts-in-wordpress/, suggests adding something like this to the theme, but obviouly it would need to be in a child theme and I'm not sure where to put it (or if it's correctly formatted for GP):

$u_time = get_the_time('U'); 
$u_modified_time = get_the_modified_time('U'); 
if ($u_modified_time >= $u_time + 86400) { 
echo "<p>Last modified on "; 
the_modified_time('F jS, Y'); 
echo " at "; 
the_modified_time(); 
echo "</p> "; } 

Thoughts? Thanks!