Does the time difference between Cairo and Singapore change during the year?

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

When is the best time to call Singapore from Cairo?

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

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

Cairo

Egypt

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Singapore

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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 Singapore.

City	Time zone	UTC offset
Cairo · Egypt	Africa/Cairo	UTC+3
Singapore · Singapore	Asia/Singapore	UTC+8

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