Counting down to 2017 will take more 1 second, e.g.
After 23.59:59 on 31 December, it will be 23:59:60
Leap seconds are a periodic one-second adjustment of Coordinated Universal Time(UTC) in order to keep a system’s time of day close to the mean solar time.
However, the Earth’s rotation speed varies in response to climatic and geological events, and due to this, UTC leap seconds are irregularly spaced and unpredictable.
For systems (redhat 4/5/6/7) not synchronized by ntpd or ptp an updated tzdata package that contains the December 31st leap second is required.
tzdata-2016g-2.el6 (redhat 6)