Image: The leap second, which is added "at that moment" in the world at 23:59:59 UT, 23:59:60 is counted as the above computer screen, the next second is counted 0:00:00 dated tomorrow. Those days, June 30 and December 31 concerned, have a duration of 86,401 seconds instead of the usual 86,400 seconds. Many applications require a still finer precision in particular GPS, fundamental physics applications and applications used in astronomy. Leap seconds can improve the accuracy of geolocation in longitude and land navigation systems as well as interoperability with other existing and future GPS systems (Glonass, Galileo, Beidou, MSAS). But this leap second poses a number of problems because some computer systems are unable to handle leap seconds, its removal is still under study.
Add or subtract one second !
Since 1972, Universal Time (UT) is the synchronized time on the rotation of the Earth. It is measured by precisely observing the passage of distant quasars to the meridian of the place of observation, with some corrections. With this method the astronomers obtain a precision of about one microsecond (10-6 second). But the rotation of the Earth on its axis, is not regular.
The International Atomic Time (IAT) is the time scale based on the definition of the second and is calculated from 349 cesium atomic clocks spread over Earth. This reference used around the world in real time, has a 10-10 second precision with cesium 133 (Cs133). This time isochronous scientific use is called the Terrestrial Time (TT).
Given the irregular rotation of the Earth, the universal time slowly drift in relation to atomic time. This drift is important in the order of 35 seconds since 1972. These microdrifts cumulatided are due to tidal effects of the Moon and Sun, to seasonal variations of ice caps, to earthquakes, to tsunamis and the different movements inner core of the Earth.
These drifts between the two clocks, UT and IAT, must be corrected, that is why since 1972, 25 seconds were added to the universal time. These seconds are called leap seconds or second additional.
Since the beginning of the measurement of IAT, the International Earth Rotation Service and Reference Systems Service (IERS) located at the Paris Observatory has only additions of leap seconds. The leap second is produced from a difference between the clocks ≈0,5 second.
NB: In 2015, in France this leap second occurred with summer time on July 1 in the morning. The clocks showed 1h 59min 59s, then 1h 59min 60s, then 2h 00min 00s. The addition of this second, correlated to variations in the rotation of our planet, is decided at the global level at the Paris Observatory within the Time-Space Reference Systems laboratory - SYRTE1
The announcement is made in advance by a newsletter, the Bulletin C, published every six months. Diffusion is arbitrated by the International Telecommunication Union.
Table: in 1958 the two clocks universal time (UT1) and International Atomic Time (IAT) are reported synchronous. In 1972, there are 10 seconds between them. This initial offset is kept but for 25 leap seconds have been added to universal time to correct its drifts. Between 31 December 1998 and 31 December 2005, the Earth has rotated regularly without drift or rather the drifts in one direction counterbalance drift in the other direction and the gap remained constant.