BA 1399 then AA 91 at Heathrow
Belfast City → London → Chicago
Timetable, London time
16:35BA 1399 from Belfast City at the gate
16:30AA 91 to Chicago leaves the gate
23 h 55 apart on the timetable, overnight
AA 91 leaves 23 h 55 after BA 1399 arrives on the timetable: more than half a day, so not a connection we count.