BA 1399 then BA 293 at Heathrow
Belfast City → London → Washington
Timetable, London time
16:35BA 1399 from Belfast City at the gate
16:25BA 293 to Washington leaves the gate
23 h 50 apart on the timetable, overnight
BA 293 leaves 23 h 50 after BA 1399 arrives on the timetable: more than half a day, so not a connection we count.