UA 3913 then UA 4261 at Houston
Denver → Houston → Lubbock
Timetable, Houston time
21:22UA 3913 from Denver at the gate
09:30UA 4261 to Lubbock leaves the gate
12 h 08 apart on the timetable, overnight
UA 4261 leaves 12 h 08 after UA 3913 arrives on the timetable: more than half a day, so not a connection we count.