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<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.

//* 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;

}