日期间隔SQL语法

Question

我在prestashop 1.7中进行此sql查询，但日期间隔不起作用。

我的代码是：

$todays =date("Y-m-d");
$t_days=date('Y-m-d', strtotime('-30 days'));

$id_clientee=$this->context->customer->id;

$spent_month  = Db::getInstance()->getValue(
        'SELECT SUM(total_paid_real) AS sum FROM ' . _DB_PREFIX_ . 'orders WHERE 

          date_add BETWEEN "'.pSQL($t_days).' 00:00:00" AND "'.pSQL($todays).' 23:59:59"
          AND id_customer = ' . (int)$id_clientee . ' 
         AND (current_state = 5 or current_state = 4 )
         ');

$spesaaaa=(int)$spent_month;

DB上的

date_add格式为：2017-05-29 13:33:13 Type: datetime

为什么“ date_add1间隔不起作用？”