setting $Local:DB::NO_LOG_QUERIES true was preventing sql trace output in QueryLogger::log_query so moved it to QueryLogger to take effect on dispatcher log() command; new Utils::datetime_to_string method + test file
* replace global xlsx_data var with class attribute to prevent data persistence when object created again in same scope * new attribute to prevent close() being called more than once on xlsx_data string, though probably no harm if it does * new test file
