UA 3913 then UA 2256 at Houston
Denver → Houston → Monterrey
Timetable, Houston time
21:22UA 3913 from Denver at the gate
10:10UA 2256 to Monterrey leaves the gate
12 h 48 apart on the timetable, overnight
UA 2256 leaves 12 h 48 after UA 3913 arrives on the timetable: more than half a day, so not a connection we count.