Does the time difference between Jakarta and London change during the year?

Yes. Because of daylight saving time the gap moves between +6h and +7h.

When is the best time to call London from Jakarta?

Between 15:00 and 17:00 in Jakarta, which is 09:00 to 11:00 in London.

Jakarta is 6h ahead of London. This gap is not fixed: daylight saving time moves it between +6h and +7h during the year.

Jakarta

Indonesia

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London

United Kingdom

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

City	Time zone	UTC offset
Jakarta · Indonesia	Asia/Jakarta	UTC+7
London · United Kingdom	Europe/London	UTC+1

What is the time difference between Jakarta and London?: Jakarta is 6h ahead of London. This gap is not fixed: daylight saving time moves it between +6h and +7h during the year.
Does the time difference between Jakarta and London change during the year?: Yes. Because of daylight saving time the gap moves between +6h and +7h.
When is the best time to call London from Jakarta?: Between 15:00 and 17:00 in Jakarta, which is 09:00 to 11:00 in London.