Athens is 6 hours ahead of Buenos Aires. Put the other way round: Buenos Aires is 6 hours behind Athens.
Buenos Aires is UTC-3 in summer and UTC-3 in winter; Athens is UTC+3 and UTC+2.
Working 09:00 to 17:00 in both cities leaves 2 hours in common: 09:00 to 11:00 in Buenos Aires, which is 15:00 to 17:00 in Athens. Those are the hours to aim a call at.
Athens changes its clocks for summer and Buenos Aires does not, so there is no single answer: the gap is 6 hours for the northern summer and 5 hours for the rest of the year.
| Buenos Aires | Athens |
|---|---|
| 08:00 | 14:00 |
| 09:00 | 15:00 |
| 10:00 | 16:00 |
| 11:00 | 17:00 |
| 12:00 | 18:00 |
| 13:00 | 19:00 |
| 14:00 | 20:00 |
| 15:00 | 21:00 |
| 16:00 | 22:00 |
| 17:00 | 23:00 |
| 18:00 | 00:00 next day |
This page covers one pair. TimeGap takes a list: add every city you care about, drag one marker, and every row reads the same instant on its own clock. It marks the hours you would actually take a call, finds the window where the most people are reachable, flags public holidays, and puts the time you pick straight into Outlook or Google Calendar.