Does the time difference between Bangkok and São Paulo change during the year?

No. The gap stays at 10h all year.

When is the best time to call São Paulo from Bangkok?

There is no overlap between standard working days, so one side has to call early or late.

Bangkok is 10h ahead of São Paulo. This gap is the same all year — neither city shifts its clocks relative to the other.

Bangkok

Thailand

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São Paulo

Brazil

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A 9-to-5 working day in Bangkok does not overlap one in São Paulo at all — somebody has to take the call outside office hours.

City	Time zone	UTC offset
Bangkok · Thailand	Asia/Bangkok	UTC+7
São Paulo · Brazil	America/Sao_Paulo	UTC−3

What is the time difference between Bangkok and São Paulo?: Bangkok is 10h ahead of São Paulo. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Bangkok and São Paulo change during the year?: No. The gap stays at 10h all year.
When is the best time to call São Paulo from Bangkok?: There is no overlap between standard working days, so one side has to call early or late.