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

No. The gap stays at 1h all year.

When is the best time to call Paris from London?

Between 09:00 and 16:00 in London, which is 10:00 to 17:00 in Paris.

London is 1h behind Paris. This gap is the same all year — neither city shifts its clocks relative to the other.

London

United Kingdom

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Paris

France

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

City	Time zone	UTC offset
London · United Kingdom	Europe/London	UTC+1
Paris · France	Europe/Paris	UTC+2

What is the time difference between London and Paris?: London is 1h behind Paris. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between London and Paris change during the year?: No. The gap stays at 1h all year.
When is the best time to call Paris from London?: Between 09:00 and 16:00 in London, which is 10:00 to 17:00 in Paris.