This year we have to compensate for a leap second before entering 2017. That means the last minute of 31st December will have 61 seconds instead of 60. Google describes how they are planning to compensate for it.
They will run the clock slower for 20 hours (ten before and ten after the last second of the year). This way they avoid glitches with systems not able to handle 61 seconds in a minute.
On the same topic, the original article links to the leap second glitch that affected Reddit.