UA 646 then WN 988 at Denver
Chicago O'Hare → Denver → Buffalo
Timetable, Denver time
15:17UA 646 from Chicago O'Hare at the gate
11:50WN 988 to Buffalo leaves the gate
20 h 33 apart on the timetable, overnight
WN 988 leaves 20 h 33 after UA 646 arrives on the timetable: more than half a day, so not a connection we count.