How can I accept an empty time?

If you wish to accept an empty time from the customer, then use this code snippet as an mu-plugin (or in your functions.php):

add_filter('openinghours_check_cart_items', 'my_openinghours_check_cart_items'); function my_openinghours_check_cart_items($r) { return empty($_POST['openinghours_time']) ? true : $r; }

This requires version 1.6.12 or later of the plugin.