SQ 1539 then AS 591 at San Francisco
Boston → San Francisco → Seattle
Timetable, San Francisco time
00:25SQ 1539 from Boston at the gate
18:48AS 591 to Seattle leaves the gatealso QF 3787
18 h 23 apart on the timetable
AS 591 leaves 18 h 23 after SQ 1539 arrives on the timetable: more than half a day, so not a connection we count.