Athens is 2.5 hours behind Chennai
Best overlap: 10:30 AM to 6:30 PM in Chennai is 8:00 AM to 4:00 PM in Athens.
Plan around 10:30 AM to 6:30 PM in Chennai, which is 8:00 AM to 4:00 PM in Athens, to catch everyone during office hours. Athens is 2.5 hours behind Chennai, but with 8 hours of common ground you can hold morning or afternoon meetings without anyone working odd hours. For part of the year the maths changes: Athens observes daylight saving, Chennai does not, shifting the overlap by an hour.