Watch Terminology Dictionary

It is a global standard time that adjusts the difference between Universal Time, which is determined by measuring the Earth's rotation, and International Primeval Time, which is measured by a cesium atomic clock. Because the Earth's rotation period is not constant, there is a difference between Universal Time and International Primeval Time. Therefore, leap seconds are inserted into International Primeval Time to adjust the difference with Universal Time to within 0.9 seconds.