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

No. The gap stays at 1h all year.

When is the best time to call Seoul from Beijing?

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

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

Beijing

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 Beijing, which is 10:00–17:00 in Seoul.

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

What is the time difference between Beijing and Seoul?: Beijing 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 Beijing and Seoul change during the year?: No. The gap stays at 1h all year.
When is the best time to call Seoul from Beijing?: Between 09:00 and 16:00 in Beijing, which is 10:00 to 17:00 in Seoul.