A3 977 then GQ 670 at Athens
Belgrade → Athens → Istanbul
Timetable, Athens time
18:05A3 977 from Belgrade at the gate
07:00GQ 670 to Istanbul leaves the gate
12 h 55 apart on the timetable, overnight
GQ 670 leaves 12 h 55 after A3 977 arrives on the timetable: more than half a day, so not a connection we count.