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

No. The gap stays at 2h all year.

When is the best time to call Nairobi from Karachi?

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

Karachi is 2h ahead of Nairobi. This gap is the same all year — neither city shifts its clocks relative to the other.

Karachi

Pakistan

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Nairobi

Kenya

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

City	Time zone	UTC offset
Karachi · Pakistan	Asia/Karachi	UTC+5
Nairobi · Kenya	Africa/Nairobi	UTC+3

What is the time difference between Karachi and Nairobi?: Karachi is 2h ahead of Nairobi. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Karachi and Nairobi change during the year?: No. The gap stays at 2h all year.
When is the best time to call Nairobi from Karachi?: Between 11:00 and 17:00 in Karachi, which is 09:00 to 15:00 in Nairobi.