The US National Institute of Standards and Technology has announced a new caesium-based atomic clock, NIST-F2 (see photo above). The clock is accurate to one second in 300 million years. Together with the older NIST-F1 clock, it will serve as the US civilian time standard – underlying time services such as the one at
time.nist.gov. The diagram below gives a conceptual view of how network time servers interconnect. Accurate clocks like NIST-F1 and NIST-F2 underpin the whole enterprise.
See also this story in Wired.