July 1 is not only the anniversary of the return of Hong Kong back to China,  it is also the day to add a  "leap second"  between 7:59:59 am and 8:00 am,  in order to match universal time.   Since the standard time used in Hong Kong is universal time plus 8 hours,  we need to delay by 1 second in order to coordinate with universal time.

It is because atmospheric motion,  in addition to other physical phenomena,  gradually slows down the rotation of Earth so that a  "leap second"  is added every few years to ensure that the difference between the different time zones in the world is less than 0.9 seconds.   This is such an amazing set up.