UA 3909 then QF 3748 at San Francisco
Cancún → San Francisco → San Diego
Timetable, San Francisco time
15:10UA 3909 from Cancún at the gate
08:20QF 3748 to San Diego leaves the gate
17 h 10 apart on the timetable, overnight
QF 3748 leaves 17 h 10 after UA 3909 arrives on the timetable: more than half a day, so not a connection we count.