function display_last_updated_date( $content ) {
$original_time = get_the_time('U');
$modified_time = get_the_modified_time('U');
if ($modified_time >= $original_time + 86400) {
$updated_time = get_the_modified_time('h:i a');
$updated_day = get_the_modified_time('F jS, Y');
$modified_content .= '<p class="last-modified">This post was last updated on '. $updated_day . ' at '. $updated_time .'</p>';
}

$modified_content .= $content;
return $modified_content;
}
add_filter( 'the_content', 'display_last_updated_date' );