BA 553 then BA 275 at Heathrow
Rome → London → Las Vegas
Timetable, London time
16:35BA 553 from Rome at the gate
16:05BA 275 to Las Vegas leaves the gate
23 h 30 apart on the timetable, overnight
BA 275 leaves 23 h 30 after BA 553 arrives on the timetable: more than half a day, so not a connection we count.