zen cart: Zen Cart 1.3.8a
PHP Version: 5.2.5 (Zend: 2.2.0)
Database: MySQL 5.0.51

I've solved this problem like this.

includes/modules/order_total/ot_coupon.php - line 251

$date_query=$db->Execute("select coupon_expire_date from " . TABLE_COUPONS . "
where coupon_expire_date >= '".date("Y-m-d G:i:s", strtotime("-1 day -1 hour"))."' and
coupon_code='" . zen_db_prepare_input($dc_check) . "'");