AA 642 then UA 5815 at O'Hare
Dallas-Fort Worth → Chicago → Lexington
Timetable, Chicago time
13:10AA 642 from Dallas-Fort Worth at the gate
10:35UA 5815 to Lexington leaves the gate
21 h 25 apart on the timetable, overnight
UA 5815 leaves 21 h 25 after AA 642 arrives on the timetable: more than half a day, so not a connection we count.