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

Yes. Because of daylight saving time the gap moves between +13h and +14h.

When is the best time to call Toronto from Seoul?

There is no overlap between standard working days, so one side has to call early or late.

Seoul is 13h ahead of Toronto. This gap is not fixed: daylight saving time moves it between +13h and +14h during the year.

Seoul

South Korea

--:--:--

Toronto

Canada

--:--:--

A 9-to-5 working day in Seoul does not overlap one in Toronto at all — somebody has to take the call outside office hours.

City	Time zone	UTC offset
Seoul · South Korea	Asia/Seoul	UTC+9
Toronto · Canada	America/Toronto	UTC−4

What is the time difference between Seoul and Toronto?: Seoul is 13h ahead of Toronto. This gap is not fixed: daylight saving time moves it between +13h and +14h during the year.
Does the time difference between Seoul and Toronto change during the year?: Yes. Because of daylight saving time the gap moves between +13h and +14h.
When is the best time to call Toronto from Seoul?: There is no overlap between standard working days, so one side has to call early or late.