UA 878 then B6 434 at San Francisco
Hong Kong → San Francisco → Boston
Timetable, San Francisco time
18:55UA 878 from Hong Kong at the gate
07:30B6 434 to Boston leaves the gatealso TP 4394, FI 7832
12 h 35 apart on the timetable, overnight
B6 434 leaves 12 h 35 after UA 878 arrives on the timetable: more than half a day, so not a connection we count.