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

No. The gap stays at 1h all year.

When is the best time to call Shanghai from Seoul?

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

Seoul is 1h ahead of Shanghai. This gap is the same all year — neither city shifts its clocks relative to the other.

Seoul

South Korea

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Shanghai

China

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

City	Time zone	UTC offset
Seoul · South Korea	Asia/Seoul	UTC+9
Shanghai · China	Asia/Shanghai	UTC+8

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