Auckland is 3 hours ahead of Seoul. Put the other way round: Seoul is 3 hours behind Auckland.
Seoul is UTC+9 in summer and UTC+9 in winter; Auckland is UTC+12 and UTC+13.
Working 09:00 to 17:00 in both cities leaves 5 hours in common: 09:00 to 14:00 in Seoul, which is 12:00 to 17:00 in Auckland. Those are the hours to aim a call at.
Auckland changes its clocks for summer and Seoul does not, so there is no single answer: the gap is 3 hours for the northern summer and 4 hours for the rest of the year.
| Seoul | Auckland |
|---|---|
| 08:00 | 11:00 |
| 09:00 | 12:00 |
| 10:00 | 13:00 |
| 11:00 | 14:00 |
| 12:00 | 15:00 |
| 13:00 | 16:00 |
| 14:00 | 17:00 |
| 15:00 | 18:00 |
| 16:00 | 19:00 |
| 17:00 | 20:00 |
| 18:00 | 21:00 |
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.