Istanbul is 1 hour ahead of Athens. Athens uses Eastern European Time (EET, UTC+2) while Istanbul uses Turkey Time (TRT, UTC+3). When it's 10:00 AM in Athens, it's 11:00 AM in Istanbul.
Greece observes daylight saving time but Turkey does not. During summer months (late March to late October), both cities are on the same time.
Any business hour works! With just 1 hour difference (or no difference in summer), scheduling calls between 9:00 AM - 5:00 PM in either city easily aligns with the other.
Athens and Istanbul are neighboring cities with rich historical connections. Direct flights take only about 1.5 hours.