UA 3875 then UA 5508 at San Francisco
IAB → San Francisco → Carlsbad
Timetable, San Francisco time
01:46UA 3875 from IAB at the gate
18:45UA 5508 to Carlsbad leaves the gate
16 h 59 apart on the timetable
UA 5508 leaves 16 h 59 after UA 3875 arrives on the timetable: more than half a day, so not a connection we count.