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

Yes. Because of daylight saving time the gap moves between −8h and −7h.

When is the best time to call Sydney from Nairobi?

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

Nairobi is 7h behind Sydney. This gap is not fixed: daylight saving time moves it between −8h and −7h during the year.

Nairobi

Kenya

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Sydney

Australia

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

City	Time zone	UTC offset
Nairobi · Kenya	Africa/Nairobi	UTC+3
Sydney · Australia	Australia/Sydney	UTC+10

What is the time difference between Nairobi and Sydney?: Nairobi is 7h behind Sydney. This gap is not fixed: daylight saving time moves it between −8h and −7h during the year.
Does the time difference between Nairobi and Sydney change during the year?: Yes. Because of daylight saving time the gap moves between −8h and −7h.
When is the best time to call Sydney from Nairobi?: Between 09:00 and 10:00 in Nairobi, which is 16:00 to 17:00 in Sydney.