UA 3913 then UA 4583 at Houston
Denver → Houston → Huntsville
Timetable, Houston time
21:22UA 3913 from Denver at the gate
09:45UA 4583 to Huntsville leaves the gate
12 h 23 apart on the timetable, overnight
UA 4583 leaves 12 h 23 after UA 3913 arrives on the timetable: more than half a day, so not a connection we count.