Does the time difference between Hong Kong and Seoul change during the year?

No. The gap stays at 1h all year.

When is the best time to call Seoul from Hong Kong?

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

Hong Kong is 1h behind Seoul. This gap is the same all year — neither city shifts its clocks relative to the other.

Hong Kong

Hong Kong SAR China

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Seoul

South Korea

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

City	Time zone	UTC offset
Hong Kong · Hong Kong SAR China	Asia/Hong_Kong	UTC+8
Seoul · South Korea	Asia/Seoul	UTC+9

What is the time difference between Hong Kong and Seoul?: Hong Kong is 1h behind Seoul. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Hong Kong and Seoul change during the year?: No. The gap stays at 1h all year.
When is the best time to call Seoul from Hong Kong?: Between 09:00 and 16:00 in Hong Kong, which is 10:00 to 17:00 in Seoul.