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

Yes. Because of daylight saving time the gap moves between +7h and +8h.

When is the best time to call Stockholm from Seoul?

Between 16:00 and 17:00 in Seoul, which is 09:00 to 10:00 in Stockholm.

Seoul is 7h ahead of Stockholm. This gap is not fixed: daylight saving time moves it between +7h and +8h during the year.

Seoul

South Korea

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Stockholm

Sweden

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A 9-to-5 day in each city overlaps for 1h. That is 16:00–17:00 in Seoul, which is 09:00–10:00 in Stockholm.

City	Time zone	UTC offset
Seoul · South Korea	Asia/Seoul	UTC+9
Stockholm · Sweden	Europe/Stockholm	UTC+2

What is the time difference between Seoul and Stockholm?: Seoul is 7h ahead of Stockholm. This gap is not fixed: daylight saving time moves it between +7h and +8h during the year.
Does the time difference between Seoul and Stockholm change during the year?: Yes. Because of daylight saving time the gap moves between +7h and +8h.
When is the best time to call Stockholm from Seoul?: Between 16:00 and 17:00 in Seoul, which is 09:00 to 10:00 in Stockholm.