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

Yes. Because of daylight saving time the gap moves between −6h and −5h.

When is the best time to call London from Chicago?

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

Chicago is 6h behind London. This gap is not fixed: daylight saving time moves it between −6h and −5h during the year.

Chicago

United States

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London

United Kingdom

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

City	Time zone	UTC offset
Chicago · United States	America/Chicago	UTC−5
London · United Kingdom	Europe/London	UTC+1

What is the time difference between Chicago and London?: Chicago is 6h behind London. This gap is not fixed: daylight saving time moves it between −6h and −5h during the year.
Does the time difference between Chicago and London change during the year?: Yes. Because of daylight saving time the gap moves between −6h and −5h.
When is the best time to call London from Chicago?: Between 09:00 and 11:00 in Chicago, which is 15:00 to 17:00 in London.