DL 641 then UA 1254 at Atlanta
Cancún → Atlanta → Denver
Timetable, Atlanta time
16:25DL 641 from Cancún at the gate
11:14UA 1254 to Denver leaves the gate
18 h 49 apart on the timetable, overnight
UA 1254 leaves 18 h 49 after DL 641 arrives on the timetable: more than half a day, so not a connection we count.