BA 521 then AA 9600 at Heathrow
Firenze → London → Chicago
Timetable, London time
13:35BA 521 from Firenze at the gate
13:00AA 9600 to Chicago leaves the gate
23 h 25 apart on the timetable, overnight
AA 9600 leaves 23 h 25 after BA 521 arrives on the timetable: more than half a day, so not a connection we count.