DL 9970 then DL 860 at San Francisco
Seattle → San Francisco → Boston
Timetable, San Francisco time
11:15DL 9970 from Seattle at the gate
15:05DL 860 to Boston leaves the gate
3 h 50 apart on the timetable
The last days
DL 9970 and DL 860 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.