Not everything is uncertain! III
The most accurate clocks in the world are “atomic clocks” that produce light of frequency n which is precisely defined beacuse energies in atoms are quantized to definite values and E = h n.
Time standard for the United States is a clock the uses Cesium atoms
History of Atomic Clocks at NIST http://www.boulder.nist.gov/timefreq/cesium/atomichistory.htm
Uncertainty less than 2 x 10-15, which means it would neither gain nor lose a second in 20 million years!