UA 1665 then DL 2426 at Boston
San Francisco → Boston → Washington National
Timetable, Boston time
07:56UA 1665 from San Francisco at the gate
18:00DL 2426 to Washington National leaves the gate
10 h 04 apart on the timetable
The last days
UA 1665 and DL 2426 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.