Another name for Daylight Saving Time, DST, is summer time (夏令). I guess nowhere in Hong Kong except some secondary schools implements summer time today.
DST works by advancing the clock by one hour or more in the summer, then adjust it back to the standard time in the autumn. By doing so, people would wake up earlier than normal during DST. Since the day time is longer in summer, people can still enjoy the daylight when it's the evening.
DST was proposed in the early twentith century to save enery for lighting during the night. However, only very few countries or cities are still having DST nowadays because it is believed to do more harms than good. For example, the energy spent on air-conditioning in the day time probably overcomes the energy saved for lighting. Statistics also showed that clock shifts would increase the risk for heart attack.
If you are interested to know more about DST, here is the Wikipedia that I referenced.
Once Upon A Day: The challenge to write one thing I learned for the day.