BA 1399 then BA 297 at Heathrow
Belfast City → London → Chicago
Timetable, London time
16:25BA 1399 from Belfast City at the gate
15:40BA 297 to Chicago leaves the gate
23 h 15 apart on the timetable, overnight
BA 297 leaves 23 h 15 after BA 1399 arrives on the timetable: more than half a day, so not a connection we count.