World Time/Leap seconds
Leap seconds are added to keep the clocks synchronized with the Earth's rotation.
The second is the base unit for modern time keeping. The second was previously defined based on the Earth's rotation, but because modern atomic clocks are more accurate than the Earth's rotation the definition was changed in 1967. A second is currently defined as being the duration of 9,192,631,770 periods/oscillations of radiation from a Cesium-133 atom at the ground state (near 0 Kelvin - coldest possible).
The Earth is rotating slower and slower over time, while the atomic clocks are not slowing down. On one average day the difference is around 0.002 seconds, which means around 1 second in 500 days. In order to synchronize the atomic clocks with the Earth's observed rotation, the atomic clocks are occasionally instructed to add an extra second – the leap second. Leap seconds are inserted so that the difference between the UTC (Coordinated Universal Time) and UT1 (mean solar time - observed Earth rotation) is kept below 0.9 seconds.
The leap second is added in the end of June or December. It is also possible to have a negative leap second, where one second is removed, in a case where the Earth is rotating faster, but such a negative second has never been used, and is rather unlikely to be used in the future.
How are leap seconds declared?
The International Earth Rotation and Reference System Service (IERS) observes the Earth's rotation and nearly 6 months in advance (January and July) a "Bulletin C" message is sent out, which reports whether or not to add a leap second in the end of June and December.
How leap seconds are inserted
Leap seconds are inserted at the end of June or December as an additional second after 23:59:59 UTC (Universal Time Coordinated). The additional second is the 61st second of the last minute of the month, and it is written as 23:59:60 (or 11:59:60 PM in 12-hour format).
The second is inserted at the same time all over the world - the actual local time will therefore depend on the time zone. Only regions in the UTC time zone will add the second just before midnight, for time zones east of UTC, the second will be added the next day (first day in January or July), for time zones west of UTC, the second will be added earlier on the same day as for UTC.
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