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

No. The gap stays at 1h all year.

When is the best time to call Singapore from Bangkok?

Between 09:00 and 16:00 in Bangkok, which is 10:00 to 17:00 in Singapore.

Bangkok is 1h behind Singapore. This gap is the same all year — neither city shifts its clocks relative to the other.

Bangkok

Thailand

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Singapore

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A 9-to-5 day in each city overlaps for 7h. That is 09:00–16:00 in Bangkok, which is 10:00–17:00 in Singapore.

City	Time zone	UTC offset
Bangkok · Thailand	Asia/Bangkok	UTC+7
Singapore · Singapore	Asia/Singapore	UTC+8

What is the time difference between Bangkok and Singapore?: Bangkok is 1h behind Singapore. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Bangkok and Singapore change during the year?: No. The gap stays at 1h all year.
When is the best time to call Singapore from Bangkok?: Between 09:00 and 16:00 in Bangkok, which is 10:00 to 17:00 in Singapore.