select unix_timestamp(now());      结果是：1618394534

select current_timestamp();          结果是：2021-04-14 18:02:14

select now();                               结果是：2021-04-14 18:02:14

实例：查询下单超过3分钟未付款的订单

 $where = " pay_status='0'   and  unix_timestamp(now())-add_time>3*60 ";

$orderlist = Db::table( 'client_order_info' )->where( $where )->order( 'order_id', 'asc' )->select();