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

Yes. Because of daylight saving time the gap moves between +1h and +2h.

When is the best time to call Stockholm from Moscow?

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

Moscow is 1h ahead of Stockholm. This gap is not fixed: daylight saving time moves it between +1h and +2h during the year.

Moscow

Russia

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Stockholm

Sweden

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A 9-to-5 day in each city overlaps for 7h. That is 10:00–17:00 in Moscow, which is 09:00–16:00 in Stockholm.

City	Time zone	UTC offset
Moscow · Russia	Europe/Moscow	UTC+3
Stockholm · Sweden	Europe/Stockholm	UTC+2

What is the time difference between Moscow and Stockholm?: Moscow is 1h ahead of Stockholm. This gap is not fixed: daylight saving time moves it between +1h and +2h during the year.
Does the time difference between Moscow and Stockholm change during the year?: Yes. Because of daylight saving time the gap moves between +1h and +2h.
When is the best time to call Stockholm from Moscow?: Between 10:00 and 17:00 in Moscow, which is 09:00 to 16:00 in Stockholm.