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

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

When is the best time to call Nairobi from London?

Between 09:00 and 15:00 in London, which is 11:00 to 17:00 in Nairobi.

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

London

United Kingdom

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Nairobi

Kenya

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

City	Time zone	UTC offset
London · United Kingdom	Europe/London	UTC+1
Nairobi · Kenya	Africa/Nairobi	UTC+3

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