UA 503 then FX 9791 at O'Hare
Boston → Chicago → Anchorage
Timetable, Chicago time
12:48UA 503 from Boston at the gate
04:00FX 9791 to Anchorage leaves the gate
15 h 12 apart on the timetable, overnight
FX 9791 leaves 15 h 12 after UA 503 arrives on the timetable: more than half a day, so not a connection we count.