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

No. Both cities keep the same local time all year.

When is the best time to call Tokyo from Seoul?

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

Seoul and Tokyo are on the same local time. This gap is the same all year — neither city shifts its clocks relative to the other.

Seoul

South Korea

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Tokyo

Japan

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A 9-to-5 day in each city overlaps for 8h. That is 09:00–17:00 in Seoul, which is 09:00–17:00 in Tokyo.

City	Time zone	UTC offset
Seoul · South Korea	Asia/Seoul	UTC+9
Tokyo · Japan	Asia/Tokyo	UTC+9

What is the time difference between Seoul and Tokyo?: Seoul and Tokyo are on the same local time. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Seoul and Tokyo change during the year?: No. Both cities keep the same local time all year.
When is the best time to call Tokyo from Seoul?: Between 09:00 and 17:00 in Seoul, which is 09:00 to 17:00 in Tokyo.