Does the time difference between Singapore and Zurich change during the year?

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

When is the best time to call Zurich from Singapore?

Between 15:00 and 17:00 in Singapore, which is 09:00 to 11:00 in Zurich.

Singapore is 6h ahead of Zurich. This gap is not fixed: daylight saving time moves it between +6h and +7h during the year.

Singapore

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Zurich

Switzerland

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

City	Time zone	UTC offset
Singapore · Singapore	Asia/Singapore	UTC+8
Zurich · Switzerland	Europe/Zurich	UTC+2

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