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

No. The gap stays at 4h all year.

When is the best time to call Istanbul from Bangkok?

Between 13:00 and 17:00 in Bangkok, which is 09:00 to 13:00 in Istanbul.

Bangkok is 4h ahead of Istanbul. This gap is the same all year — neither city shifts its clocks relative to the other.

Bangkok

Thailand

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Istanbul

Türkiye

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

City	Time zone	UTC offset
Bangkok · Thailand	Asia/Bangkok	UTC+7
Istanbul · Türkiye	Europe/Istanbul	UTC+3

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