While humans measure time in regular intervals in minutes and seconds; months and years, the earth’s orbit is more irregular than that. Better observation and time-keeping methods went away from using the revolutions around the sun as a guidepost and to more precise atom oscillations. The difference between the two added an extra 0.6 seconds to a solar year. So in the tradition of a leap day, scientists added a leap second.

On this day, June 30, in 1972, The International Earth Rotation and Reference Systems Service announced the addition of the first leap second to the clocks.

Unlike the leap year, there is no set recurrence for the second, as the Earth’s orbit is too unpredictable. Scientists continually monitor the difference between the atomic and solar time time, announcing six months in advance the addition of a leap second whenever that difference approaches 0.6. seconds. The last time that happened was in 2008. The next day with an extra second was announced to be June 30, 2012.