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

Yes. Because of daylight saving time the gap moves between +17h and +19h.

When is the best time to call Vancouver from Sydney?

Between 09:00 and 10:00 in Sydney, which is 16:00 to 17:00 in Vancouver.

Sydney is 17h ahead of Vancouver. This gap is not fixed: daylight saving time moves it between +17h and +19h during the year.

Sydney

Australia

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Vancouver

Canada

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A 9-to-5 day in each city overlaps for 1h. That is 09:00–10:00 in Sydney, which is 16:00–17:00 in Vancouver.

City	Time zone	UTC offset
Sydney · Australia	Australia/Sydney	UTC+10
Vancouver · Canada	America/Vancouver	UTC−7

What is the time difference between Sydney and Vancouver?: Sydney is 17h ahead of Vancouver. This gap is not fixed: daylight saving time moves it between +17h and +19h during the year.
Does the time difference between Sydney and Vancouver change during the year?: Yes. Because of daylight saving time the gap moves between +17h and +19h.
When is the best time to call Vancouver from Sydney?: Between 09:00 and 10:00 in Sydney, which is 16:00 to 17:00 in Vancouver.