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

Yes. Because of daylight saving time the gap moves between −3h and −2h.

When is the best time to call Sydney from Singapore?

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

Singapore is 2h behind Sydney. This gap is not fixed: daylight saving time moves it between −3h and −2h during the year.

Singapore

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Sydney

Australia

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A 9-to-5 day in each city overlaps for 6h. That is 09:00–15:00 in Singapore, which is 11:00–17:00 in Sydney.

City	Time zone	UTC offset
Singapore · Singapore	Asia/Singapore	UTC+8
Sydney · Australia	Australia/Sydney	UTC+10

What is the time difference between Singapore and Sydney?: Singapore is 2h behind Sydney. This gap is not fixed: daylight saving time moves it between −3h and −2h during the year.
Does the time difference between Singapore and Sydney change during the year?: Yes. Because of daylight saving time the gap moves between −3h and −2h.
When is the best time to call Sydney from Singapore?: Between 09:00 and 15:00 in Singapore, which is 11:00 to 17:00 in Sydney.