Tokyo is 1 hour ahead of Hong Kong. Put the other way round: Hong Kong is 1 hour behind Tokyo.
Hong Kong is UTC+8 in summer and UTC+8 in winter; Tokyo is UTC+9 and UTC+9.
Working 09:00 to 17:00 in both cities leaves 7 hours in common: 09:00 to 16:00 in Hong Kong, which is 10:00 to 17:00 in Tokyo. Those are the hours to aim a call at.
Neither Hong Kong nor Tokyo changes its clocks, so this gap is the same every day of the year.
| Hong Kong | Tokyo |
|---|---|
| 08:00 | 09:00 |
| 09:00 | 10:00 |
| 10:00 | 11:00 |
| 11:00 | 12:00 |
| 12:00 | 13:00 |
| 13:00 | 14:00 |
| 14:00 | 15:00 |
| 15:00 | 16:00 |
| 16:00 | 17:00 |
| 17:00 | 18:00 |
| 18:00 | 19:00 |
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.