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

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

When is the best time to call Toronto from Paris?

Between 15:00 and 17:00 in Paris, which is 09:00 to 11:00 in Toronto.

Paris is 6h ahead of Toronto. This gap is not fixed: daylight saving time moves it between +5h and +6h during the year.

Paris

France

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Toronto

Canada

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

City	Time zone	UTC offset
Paris · France	Europe/Paris	UTC+2
Toronto · Canada	America/Toronto	UTC−4

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