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

Yes. Because of daylight saving time the gap moves between −7h and −6h.

When is the best time to call Shanghai from Berlin?

Between 09:00 and 11:00 in Berlin, which is 15:00 to 17:00 in Shanghai.

Berlin is 6h behind Shanghai. This gap is not fixed: daylight saving time moves it between −7h and −6h during the year.

Berlin

Germany

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Shanghai

China

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A 9-to-5 day in each city overlaps for 2h. That is 09:00–11:00 in Berlin, which is 15:00–17:00 in Shanghai.

City	Time zone	UTC offset
Berlin · Germany	Europe/Berlin	UTC+2
Shanghai · China	Asia/Shanghai	UTC+8

What is the time difference between Berlin and Shanghai?: Berlin is 6h behind Shanghai. This gap is not fixed: daylight saving time moves it between −7h and −6h during the year.
Does the time difference between Berlin and Shanghai change during the year?: Yes. Because of daylight saving time the gap moves between −7h and −6h.
When is the best time to call Shanghai from Berlin?: Between 09:00 and 11:00 in Berlin, which is 15:00 to 17:00 in Shanghai.