protected function getPaymentDateOrder()
    {
        // Take date of last payment as payment date.
        $paymentDate = null;
        foreach ($this->source->getStatusHistoryCollection() as $statusChange) {
            /** @var \Mage_Sales_Model_Order_Status_History $statusChange */
            if (!$paymentDate || $this->isPaidStatus($statusChange->getStatus())) {
                $createdAt = substr($statusChange->getCreatedAt(), 0, strlen('yyyy-mm-dd'));
                if (!$paymentDate || $createdAt < $paymentDate) {
                    $paymentDate = $createdAt;
                }
            }
        }
        return $paymentDate;
    }