Does the time difference between Seoul and Sydney change during the year?

Yes. Because of daylight saving time the gap moves between −2h and −1h.

When is the best time to call Sydney from Seoul?

Between 09:00 and 16:00 in Seoul, which is 10:00 to 17:00 in Sydney.

Seoul is 1h behind Sydney. This gap is not fixed: daylight saving time moves it between −2h and −1h during the year.

Seoul

South Korea

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Sydney

Australia

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A 9-to-5 day in each city overlaps for 7h. That is 09:00–16:00 in Seoul, which is 10:00–17:00 in Sydney.

City	Time zone	UTC offset
Seoul · South Korea	Asia/Seoul	UTC+9
Sydney · Australia	Australia/Sydney	UTC+10

What is the time difference between Seoul and Sydney?: Seoul is 1h behind Sydney. This gap is not fixed: daylight saving time moves it between −2h and −1h during the year.
Does the time difference between Seoul and Sydney change during the year?: Yes. Because of daylight saving time the gap moves between −2h and −1h.
When is the best time to call Sydney from Seoul?: Between 09:00 and 16:00 in Seoul, which is 10:00 to 17:00 in Sydney.