UA 3874 then AA 2553 at O'Hare
Chicago Rockford → Chicago → Miami
Timetable, Chicago time
01:21UA 3874 from Chicago Rockford at the gate
16:31AA 2553 to Miami leaves the gate
15 h 10 apart on the timetable
AA 2553 leaves 15 h 10 after UA 3874 arrives on the timetable: more than half a day, so not a connection we count.