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

Yes. Because of daylight saving time the gap moves between −4h and −3h.

When is the best time to call Sydney from Jakarta?

Between 09:00 and 14:00 in Jakarta, which is 12:00 to 17:00 in Sydney.

Jakarta is 3h behind Sydney. This gap is not fixed: daylight saving time moves it between −4h and −3h during the year.

Jakarta

Indonesia

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Sydney

Australia

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A 9-to-5 day in each city overlaps for 5h. That is 09:00–14:00 in Jakarta, which is 12:00–17:00 in Sydney.

City	Time zone	UTC offset
Jakarta · Indonesia	Asia/Jakarta	UTC+7
Sydney · Australia	Australia/Sydney	UTC+10

What is the time difference between Jakarta and Sydney?: Jakarta is 3h behind Sydney. This gap is not fixed: daylight saving time moves it between −4h and −3h during the year.
Does the time difference between Jakarta and Sydney change during the year?: Yes. Because of daylight saving time the gap moves between −4h and −3h.
When is the best time to call Sydney from Jakarta?: Between 09:00 and 14:00 in Jakarta, which is 12:00 to 17:00 in Sydney.