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

No. The gap stays at 6h all year.

When is the best time to call Tokyo from Istanbul?

Between 09:00 and 11:00 in Istanbul, which is 15:00 to 17:00 in Tokyo.

Istanbul is 6h behind Tokyo. This gap is the same all year — neither city shifts its clocks relative to the other.

Istanbul

Türkiye

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Tokyo

Japan

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A 9-to-5 day in each city overlaps for 2h. That is 09:00–11:00 in Istanbul, which is 15:00–17:00 in Tokyo.

City	Time zone	UTC offset
Istanbul · Türkiye	Europe/Istanbul	UTC+3
Tokyo · Japan	Asia/Tokyo	UTC+9

What is the time difference between Istanbul and Tokyo?: Istanbul is 6h behind Tokyo. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Istanbul and Tokyo change during the year?: No. The gap stays at 6h all year.
When is the best time to call Tokyo from Istanbul?: Between 09:00 and 11:00 in Istanbul, which is 15:00 to 17:00 in Tokyo.