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

Yes. Because of daylight saving time the gap moves between −8h and −7h.

When is the best time to call Seoul from Berlin?

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

Berlin is 7h behind Seoul. This gap is not fixed: daylight saving time moves it between −8h and −7h during the year.

Berlin

Germany

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Seoul

South Korea

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A 9-to-5 day in each city overlaps for 1h. That is 09:00–10:00 in Berlin, which is 16:00–17:00 in Seoul.

City	Time zone	UTC offset
Berlin · Germany	Europe/Berlin	UTC+2
Seoul · South Korea	Asia/Seoul	UTC+9

What is the time difference between Berlin and Seoul?: Berlin is 7h behind Seoul. This gap is not fixed: daylight saving time moves it between −8h and −7h during the year.
Does the time difference between Berlin and Seoul change during the year?: Yes. Because of daylight saving time the gap moves between −8h and −7h.
When is the best time to call Seoul from Berlin?: Between 09:00 and 10:00 in Berlin, which is 16:00 to 17:00 in Seoul.