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However, for the past several centuries, the length of the mean solar day has been increasing by about 1.4–1.7 ms per century, depending on the averaging time.
Therefore, time standards that change the date after precisely 86,400 SI seconds, such as the International Atomic Time (TAI), will get increasingly ahead of time standards tied to the mean solar day, such as Greenwich Mean Time (GMT).
Specifically, a positive leap second is inserted between second of a chosen UTC calendar date and second of the following date.
The definition of UTC states that the last day of December and June are preferred, with the last day of March or September as second preference, and the last day of any other month as third preference.
Therefore, if the UTC day were defined as precisely 86400 SI seconds, the UTC time-of-day would slowly drift apart from that of solar-based standards, such as Greenwich Mean Time (GMT) and its successor UT1.
The purpose of a leap second is to compensate for this drift, by occasionally scheduling some UTC days with 86401 or (in principle) 86399 SI seconds.
When the Coordinated Universal Time standard was instituted in 1961, based on atomic clocks, it was felt necessary to maintain agreement with the GMT time of day, which, until then, had been the reference for broadcast time services.
Thus, from 1961 to 1971, the rate of (some) atomic clocks was constantly slowed to remain synchronised with GMT.
In 1972, the leap-second system was introduced so that the broadcast UTC seconds could be made exactly equal to the standard SI second, while still maintaining the UTC time of day and changes of UTC date synchronized with those of UT1 (the solar time standard that superseded GMT).
All leap seconds (as of 2017) have been scheduled for either June 30 or December 31. On clocks that display local time tied to UTC, the leap second may be inserted at the end of some other hour (or half-hour or quarter-hour), depending on the local time zone.
A negative leap second would suppress second of the last day of a chosen month, so that second of that date would be followed immediately by second of the following date.
Unlike leap days, UTC leap seconds occur simultaneously worldwide; for example, the leap second on December 31, 2005 UTC was December 31, 2005 ( p.m.) in U. Eastern Standard Time and January 1, 2006 (a.m.) in Japan Standard Time.
Not all clocks implement leap seconds in the same manner as UTC.
Leap seconds in Unix time are commonly implemented by repeating the last second of the day.