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

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

When is the best time to call Nairobi from Melbourne?

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

Melbourne is 7h ahead of Nairobi. This gap is not fixed: daylight saving time moves it between +7h and +8h during the year.

Melbourne

Australia

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Nairobi

Kenya

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

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

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