UA 3883 then B6 639 at Boston
San Francisco → Boston → Austin
Timetable, Boston time
08:16UA 3883 from San Francisco at the gate
12:05B6 639 to Austin leaves the gate
3 h 49 apart on the timetable
The last days
UA 3883 and B6 639 weren’t both flown 15 min to 12 h apart in the last 21 days.
Statistics from past flights, not live flight information. Always check times and your connection with your airline, and don’t base travel or booking decisions on these numbers.