When is next leap second?

Leap seconds are necessary because time is measured utilizing stable atomic clocks (TAI or International Atomic Time), whereas the rotation of the Earth has been slowing down. Traditionally, the second has been defined as 1/86400 of a mean solar day (see solar time). This is determined by the rotation of the Earth around its axis and its orbit around the Sun; time was measured by astronomical observations. However, the solar day has gradually become 1.7 ms longer every century, due mainly to the tidal acceleration of the Moon. The SI second that is counted by atomic time standards has been defined in such a way that its length matched the nominal second of 1/86400 of a mean solar day between 1750 and 1892. Since that time the length of the solar day has been slowly increasing. Therefore the time as measured by the rotation of the Earth has been accumulating a delay with respect to atomic time standards. From 1961 to 1971 the rate of atomic clocks was constantly slowed down in order to stay in sync with the rotation of the Earth (before 1961, broadcast time was synchronized to astronomically determined Greenwich Mean Time). From 1972 onwards, broadcast seconds have been exactly equal to the length of the SI second chosen in 1967 as a certain number of atomic vibrations. UTC is counted by atomic clocks, but is kept approximately in sync with UT1 (mean solar time) by introducing a leap second whenever necessary. This happens when the difference UT1−UTC is approaching 0.9 seconds, and is scheduled either between 30 June and 1 July of a year, or between 31 December of the current and 1 January of the next year. On January 1, 1972, the initial offset of UTC from TAI was chosen to be 10 seconds, which approximated the total difference which had accumulated between UT1 and TAI since 1958, when TAI was defined equal to UT1 (GMT). The table above shows the number of leap seconds added since then. The total difference between TAI and UTC is 10 seconds more than the total number of leap seconds.