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

No. The gap stays at 2h all year.

When is the best time to call Seoul from Bangkok?

Between 09:00 and 15:00 in Bangkok, which is 11:00 to 17:00 in Seoul.

Bangkok is 2h behind Seoul. This gap is the same all year — neither city shifts its clocks relative to the other.

Bangkok

Thailand

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Seoul

South Korea

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

City	Time zone	UTC offset
Bangkok · Thailand	Asia/Bangkok	UTC+7
Seoul · South Korea	Asia/Seoul	UTC+9

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