Does the time difference between Hong Kong and Singapore change during the year?

No. Both cities keep the same local time all year.

When is the best time to call Singapore from Hong Kong?

Between 09:00 and 17:00 in Hong Kong, which is 09:00 to 17:00 in Singapore.

Hong Kong and Singapore are on the same local time. This gap is the same all year — neither city shifts its clocks relative to the other.

Hong Kong

Hong Kong SAR China

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Singapore

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A 9-to-5 day in each city overlaps for 8h. That is 09:00–17:00 in Hong Kong, which is 09:00–17:00 in Singapore.

City	Time zone	UTC offset
Hong Kong · Hong Kong SAR China	Asia/Hong_Kong	UTC+8
Singapore · Singapore	Asia/Singapore	UTC+8

What is the time difference between Hong Kong and Singapore?: Hong Kong and Singapore are on the same local time. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Hong Kong and Singapore change during the year?: No. Both cities keep the same local time all year.
When is the best time to call Singapore from Hong Kong?: Between 09:00 and 17:00 in Hong Kong, which is 09:00 to 17:00 in Singapore.