It is about time - NIST-F2

Great article for Time Nuts at Slate:

Just a Second
The leap second may be a ticking time bomb.
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 cylinder of a platinum-iridium alloy entombed at the NIST campus near Washington, D.C. is the kilogram, the pulses delivered by this clock make it not merely a timepiece but the timepiece; all others are calibrated against it.*

For decades, the world’s primary timescale, Universal Coordinated Time, has been hitched to the numbers put forth by a network of atomic clocks scattered around the globe, including NIST-F2. Because NIST-F2 is more precise than other atomic clocks, it’s reasonable to say that it is the most accurate clock humanity has ever built. It also happens to be more accurate at keeping time than Mother Nature, a statement that is both true and controversial, for reasons we will get to in a carefully clocked minute.

NIST-F2 works by fiddling with the spin of electrons within cesium atoms. The 12-foot-tall machine uses lasers to corral about 100 million cesium atoms into a tight configuration that slows their motion and cools them to nearly absolute zero. More lasers nudge the cluster up through a microwave chamber, and then gravity brings it back down again. The carefully tuned microwave radiation changes the state of the cesium atoms. In the 1950s, scientists calculated the number of cycles of this radiation that occur during the most precise astronomical definition of a second. That number is 9,192,631,770, which looks like digit soup to most of us, but time nerds know it instantly. A second, at least in the standards sense of it, is not one-sixtieth of a minute or even one of 86,400 parts of a day.  It is “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.” 

The Leapsecond:

At issue is the blending of two timescales. Time tracked by atomic clocks is diverging from time as determined by that most basic of timepieces: the rotating Earth. Whereas the best atomic clocks today have an error rate that translates into roughly one misshapen second every 100 million years, Earth, by comparison, is unreliable. That’s because the planet’s rotation is irregular and gradually slowing down. (It may feel like life passes by in a blink, but days are actually getting longer, albeit slowly.) This gradual deceleration, mixed with shorter-term changes to Earth’s rotational velocity, make the heavens a less than trusty timekeeper.

A nice long (three pages) article talking about accurate timekeeping and how crucial it is to our modern society.

I maintain a few clocks here that are very accurate - these were developed by the cellular telephone companies to be able to triangulate the location of 911 calls so the timing is accurate down to a couple hundred nanoseconds. As newer clocks come out, the old ones are sold on the surplus market for pennies on the dollar but they are still perfectly functional as equipment built for any telephone service is built like a tank and essentially bulletproof.