You can order by multiple things see more on https://make.wordpress.org/core/2014/08/29/a-more-powerful-order-by-in-wordpress-4-0/:
$args = array(
'meta_key' => 'start_time',
'orderby' => array('date' => 'ASC', 'meta_value' => 'ASC)
);
You can order by multiple things see more on https://make.wordpress.org/core/2014/08/29/a-more-powerful-order-by-in-wordpress-4-0/:
$args = array(
'meta_key' => 'start_time',
'orderby' => array('date' => 'ASC', 'meta_value' => 'ASC)
);