'Return the datetime value to use as Schedule Date (``date_planned``) for
PO Lines that correspond to the given product.seller_ids,
when ordered at `date_order_str`.
:param browse_record | False product: product.product, used to
determine delivery delay thanks to the selected seller field (if False, default delay = 0)
:param browse_record | False po: purchase.order, necessary only if
the PO line is not yet attached to a PO.
:rtype: datetime
:return: desired Schedule Date for the PO line'

@api.model
def _get_date_planned(self, seller, po=False):

    date_order = (po.date_order if po else self.order_id.date_order)
    if date_order:
        return (datetime.strptime(date_order, DEFAULT_SERVER_DATETIME_FORMAT) + relativedelta(days=(seller.delay if seller else 0)))
    else:
        return (datetime.today() + relativedelta(days=(seller.delay if seller else 0)))
