Leap seconds: By international convention, UTC (which is an arbitrary human invention) is kept within 0.9 seconds of physical reality (UT1, which is a measure of solar time) by introducing a "leap second" in the last minute of the UTC year, or in the last minute of June. [The deviation from UT1 varies "erratically" with changes in things like positions of planets, tides, weather, and ocean currents, which cause slight in the mass/angular momentum distribution of the earth.]I can cope.
Leap seconds don't have to be announced much more than six months before they happen. This is a problem if you need second-accurate planning beyond six months.
(from http://unix4lyfe.org/time)
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