BA 581 then BA 636 at Heathrow
Milan → London → Athens
Timetable, London time
13:25BA 581 from Milan at the gate
13:10BA 636 to Athens leaves the gate
23 h 45 apart on the timetable, overnight
BA 636 leaves 23 h 45 after BA 581 arrives on the timetable: more than half a day, so not a connection we count.