Zkuste toto -
SELECT
s1.planned_arrival - s2.planned_arrival AS val
FROM
shipment_stop s1,
shipment_stop s2
WHERE
s1.stop_num = 1
AND s2.stop_num = 5
AND s1.shipment_gid = 'IFFCO/LOGISTICS.L171009358'
AND s1.shipment_gid = s2.shipment_gid;