UA 3901 then B6 1034 at San Francisco
Hong Kong → San Francisco → Boston
Timetable, San Francisco time
16:34UA 3901 from Hong Kong at the gate
12:14B6 1034 to Boston leaves the gate
19 h 40 apart on the timetable, overnight
B6 1034 leaves 19 h 40 after UA 3901 arrives on the timetable: more than half a day, so not a connection we count.