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

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

When is the best time to call Madrid from Bangkok?

Between 14:00 and 17:00 in Bangkok, which is 09:00 to 12:00 in Madrid.

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

Bangkok

Thailand

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Madrid

Spain

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

City	Time zone	UTC offset
Bangkok · Thailand	Asia/Bangkok	UTC+7
Madrid · Spain	Europe/Madrid	UTC+2

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