正闰秒１９６７年国际计量局决定从当年１０月起，国际标准时间将由原子时（ＡＴ）ＩＳ秒连续计数求得。其秒定义为“铯原子Ｃｓ１３３基态的两个超精细能级间跃迁辐射的９１９２６３１７７０个周期所持续的时间为１秒。”又经国际协议决定于１９６８年１月０时０分０秒作... Positive leap seconds In 1967, the International Bureau of Weights and Measures decided that from October of that year, the international standard time will be obtained by continuous counting of Atomic Time (AT) seconds. The second is defined as “the duration of 9192631770 cycles of the transition between the two hyperfine levels of the Cs133 ground state of cesium atom is 1 second.” It was also determined by international agreement at 0:00:010 January 0, 1968. ..