On December 31, 2016 at 11:59 PM and 59 seconds Universal Time you will benefit from an extra second. Fortunately in California, The official time will be 3:59 and 60 seconds before the clock strikes 4:00pm (Locally). This will happen at 7:00pm on New Years Eve in New York Times Square.
The last time this happened was June 30, 2015. Atomic clocks keep very accurate time, however the rotation of the earth is affected by tides, the moon and other planets. Scientists can track the change in the rotation of the earth and make adjustments when the difference becomes larger than half a second. See the related web page or search Wikipedia for the full story behind the leap second. Make good use of your extra second and find a cache.