Shop_coupon: make the expiry date column sortable

I’m trying to make the expiry_date column sortable.

I guess the problem has something to do with an incorrect meta key value.

function my_sortable_expiry_date_column( $columns ) {
    $columns['expiry_date'] = 'expiry_date';

    //To make a column 'un-sortable' remove it from the array
    //unset($columns['date']);

    return $columns;
}
add_filter( 'manage_edit-shop_coupon_sortable_columns', 'my_sortable_expiry_date_column' );

function my_expiry_date_orderby( $query ) {
    if( ! is_admin() )
        return;

    $orderby = $query->get( 'orderby');

    if($orderby == 'expiry_date') {
        $query->set('meta_key','expiry_date');
        $query->set('orderby','meta_value_num');
    }
}
add_action( 'pre_get_posts', 'my_expiry_date_orderby' );

How can I find the correct meta_key? How do I orberby ‘date types’ column?

Thnx!