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

No. The gap stays at 6h all year.

When is the best time to call Seoul from Moscow?

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

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

Moscow

Russia

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Seoul

South Korea

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

City	Time zone	UTC offset
Moscow · Russia	Europe/Moscow	UTC+3
Seoul · South Korea	Asia/Seoul	UTC+9

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