UA 3874 then AA 6487 at O'Hare
Chicago Rockford → Chicago → St Louis
Timetable, Chicago time
01:21UA 3874 from Chicago Rockford at the gate
15:18AA 6487 to St Louis leaves the gate
13 h 57 apart on the timetable
AA 6487 leaves 13 h 57 after UA 3874 arrives on the timetable: more than half a day, so not a connection we count.