Does the time difference between Cairo and Hong Kong change during the year?

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

When is the best time to call Hong Kong from Cairo?

Between 09:00 and 12:00 in Cairo, which is 14:00 to 17:00 in Hong Kong.

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

Cairo

Egypt

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Hong Kong

Hong Kong SAR China

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A 9-to-5 day in each city overlaps for 3h. That is 09:00–12:00 in Cairo, which is 14:00–17:00 in Hong Kong.

City	Time zone	UTC offset
Cairo · Egypt	Africa/Cairo	UTC+3
Hong Kong · Hong Kong SAR China	Asia/Hong_Kong	UTC+8

What is the time difference between Cairo and Hong Kong?: Cairo is 5h behind Hong Kong. This gap is not fixed: daylight saving time moves it between −6h and −5h during the year.
Does the time difference between Cairo and Hong Kong change during the year?: Yes. Because of daylight saving time the gap moves between −6h and −5h.
When is the best time to call Hong Kong from Cairo?: Between 09:00 and 12:00 in Cairo, which is 14:00 to 17:00 in Hong Kong.