http://www.slate.com/articles/health_and_science/science/2015/06/next_leap_second_june_30_dangers_to_software_military_gps_banking_air_traffic.html

The most accurate tool on the planet for plotting the tick-tock of life goes by the humdrum name of NIST-F2. It is a cesium fountain atomic clock at the National Institute of Standards and Technology in Boulder, Colorado. In the same way that a single meter-long rod entombed somewhere in Paris is the meter, the pulses delivered by this clock make it not merely a timepiece but the timepiece; all others are calibrated against it.