The problem is that the Earth is very gradually slowing down, continually throwing the two timescales out of synch, so every so often, a "leap second" has to be tacked on to the atomic clock.

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Do you ever feel like some days drag on longer than others? That feeling may be psychological, but actual day length really does fluctuate--by a fraction of a millisecond. (A millisecond is one thousandth of a second)

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Now scientists are investigating how events in a mineral layer at the core-mantle boundary, 1,615 miles (2,600 kilometers) deep, similarly affect the planet's spin.

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