adding a custom time class to in theme twenty sixteen

Twentysixteen’s header and functions.php appear to support the body_class filter.

add_filter('body_class', 'my_body_classes');

function my_body_classes($classes) {

        global $wp_query;

        $time = time() + ( get_option( 'gmt_offset' ) * 3600 );
        $classes[] = 'w' . gmdate( 'W', $time );

        if ( is_page() && $wp_query->post->post_parent ){
             $classes[] = 'parent-' . $wp_query->post->post_parent;
        }

        return $classes;
}