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

No. The gap stays at 3h all year.

When is the best time to call Singapore from Karachi?

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

Karachi is 3h behind Singapore. This gap is the same all year — neither city shifts its clocks relative to the other.

Karachi

Pakistan

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Singapore

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

City	Time zone	UTC offset
Karachi · Pakistan	Asia/Karachi	UTC+5
Singapore · Singapore	Asia/Singapore	UTC+8

What is the time difference between Karachi and Singapore?: Karachi is 3h behind Singapore. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Karachi and Singapore change during the year?: No. The gap stays at 3h all year.
When is the best time to call Singapore from Karachi?: Between 09:00 and 14:00 in Karachi, which is 12:00 to 17:00 in Singapore.