AA 3974 then UA 3376 at O'Hare
Harrisburg → Chicago → St Louis
Timetable, Chicago time
15:34AA 3974 from Harrisburg at the gate
11:00UA 3376 to St Louis leaves the gate
19 h 26 apart on the timetable, overnight
UA 3376 leaves 19 h 26 after AA 3974 arrives on the timetable: more than half a day, so not a connection we count.