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

No. Both cities keep the same local time all year.

When is the best time to call Singapore from Shanghai?

Between 09:00 and 17:00 in Shanghai, which is 09:00 to 17:00 in Singapore.

Shanghai and Singapore are on the same local time. This gap is the same all year — neither city shifts its clocks relative to the other.

Shanghai

China

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Singapore

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A 9-to-5 day in each city overlaps for 8h. That is 09:00–17:00 in Shanghai, which is 09:00–17:00 in Singapore.

City	Time zone	UTC offset
Shanghai · China	Asia/Shanghai	UTC+8
Singapore · Singapore	Asia/Singapore	UTC+8

What is the time difference between Shanghai and Singapore?: Shanghai and Singapore are on the same local time. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Shanghai and Singapore change during the year?: No. Both cities keep the same local time all year.
When is the best time to call Singapore from Shanghai?: Between 09:00 and 17:00 in Shanghai, which is 09:00 to 17:00 in Singapore.