leap second : Java Glossary

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leap second
BIPM (Bureau International des Poids et Measures) in Sèvres France maintains the UTC (Coordinated Universal Time/Temps Universel Coordonné) time standard atomic clock. It needs to be adjusted periodically to keep it in sync with the clock based on the earth’s rotation, because the earth is gradually slowing down. Unlike leap years, there is no formula to predict when they will occur. Scientists measure the discrepancy and announce a leap second whenever the two clocks get too badly out of sync. This happen about once every 3 to 7 years. They always add the leap second at midnight to delay the start of the following day by one second. Midnight UTC, strictly 23:59:59 is either omitted or followed by 23:59:60. The day with the leap second contains 24 hours and one second.

Because leap seconds can’t be predicted, Java ignores them entirely. You deal with them by readjusting your computer clocks from precise atomic clocks on the web that know about the leap seconds, then pretending they never happened. For most civil purposes, this suffices. You could not get away with that sloppiness if you were implementing GPS (Global Positioning System), plotting satellite orbits, or doing astronomy.

GPS uses a SI (Systèm Internationale (metric)) clock without the leap seconds.

Leap seconds are bailing wire. They foul technology. The problem is, every once is a while you get two seconds with the same label, much the way in fall you get two hours from 1AM to 2AM confounding schedules and labour records. Communication satellites are confused and thrown off base for a few seconds. The International Bureau of Standards is considering abolishing the leap second, replacing it with continuous clock adjustment. I gather satellites would have two clocks, one unadjusted atomic and one continuously synchronised to the earth’s rotation.


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