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

No. The gap stays at 4h all year.

When is the best time to call Tokyo from Karachi?

Between 09:00 and 13:00 in Karachi, which is 13:00 to 17:00 in Tokyo.

Karachi is 4h behind Tokyo. This gap is the same all year — neither city shifts its clocks relative to the other.

Karachi

Pakistan

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Tokyo

Japan

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A 9-to-5 day in each city overlaps for 4h. That is 09:00–13:00 in Karachi, which is 13:00–17:00 in Tokyo.

City	Time zone	UTC offset
Karachi · Pakistan	Asia/Karachi	UTC+5
Tokyo · Japan	Asia/Tokyo	UTC+9

What is the time difference between Karachi and Tokyo?: Karachi is 4h behind Tokyo. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Karachi and Tokyo change during the year?: No. The gap stays at 4h all year.
When is the best time to call Tokyo from Karachi?: Between 09:00 and 13:00 in Karachi, which is 13:00 to 17:00 in Tokyo.