UA 3874 then AA 3141 at O'Hare
Chicago → Chicago → San Francisco
Timetable, Chicago time
01:21UA 3874 from Chicago at the gate
17:10AA 3141 to San Francisco leaves the gate
15 h 49 apart on the timetable
AA 3141 leaves 15 h 49 after UA 3874 arrives on the timetable: more than half a day, so not a connection we count.