UA 574 then UA 3900 at San Francisco
Boston → San Francisco → Hong Kong
Timetable, San Francisco time
12:49UA 574 from Boston at the gatealso VA 8520
07:00UA 3900 to Hong Kong leaves the gate
18 h 11 apart on the timetable, overnight
UA 3900 leaves 18 h 11 after UA 574 arrives on the timetable: more than half a day, so not a connection we count.