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

Yes. Because of daylight saving time the gap moves between −7h and −6h.

When is the best time to call Singapore from Madrid?

Between 09:00 and 11:00 in Madrid, which is 15:00 to 17:00 in Singapore.

Madrid is 6h behind Singapore. This gap is not fixed: daylight saving time moves it between −7h and −6h during the year.

Madrid

Spain

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Singapore

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

City	Time zone	UTC offset
Madrid · Spain	Europe/Madrid	UTC+2
Singapore · Singapore	Asia/Singapore	UTC+8

What is the time difference between Madrid and Singapore?: Madrid is 6h behind Singapore. This gap is not fixed: daylight saving time moves it between −7h and −6h during the year.
Does the time difference between Madrid and Singapore change during the year?: Yes. Because of daylight saving time the gap moves between −7h and −6h.
When is the best time to call Singapore from Madrid?: Between 09:00 and 11:00 in Madrid, which is 15:00 to 17:00 in Singapore.