Leap second: when time stands still

Published: 2005-10-31
Last Updated: 2005-10-31 16:39:57 UTC
by Dan Goldberg (Version: 2)
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Just after we in the US changed our clocks back an hour from daylight saving time to standard time I ran across an interesting tid-bit regarding time. This may have impacts across many information systems.
                      UTC TIME STEP
on the 1st of January 2006

A positive leap second will be introduced at the end of December 2005.
The sequence of dates of the UTC second markers will be:

2005 December 31, 23h 59m 59s
2005 December 31, 23h 59m 60s
2006 January 1, 0h 0m 0s

see: http://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat

This is being done to keep time in sync with the slowing earths rotation and the standard time measures in use in atomic clocks. There have been several leap seconds introduced since 1972. My original source for this information is the November 2005 Scientific American article by Wendy Grossman.  The impacts on information systems can come from GPS clocks not recognizing the leap second and  sending out flawed or inaccurate data with can affect many time based functions including security  features.  According to the article there is talk within the INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS SERVICE (IERS) (A united nations organization) of decoupling standard time measurement from the earth's rotation and adhering strictly to atomic decay.

NTP systems appear to be able to handle leap seconds: http://www.eecis.udel.edu/~mills/leap.html

Scientific American article here.

Happy travels through time ...
Dan Goldberg
Dan at madjic dot net

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