• June 25, 2022

Are you ready for the “Leap Second?”

Leap SecondDid you know that a “leap second” will be added to our clocks at midnight on June 30th 2015? What is that all about?

The rotation of the earth around its axis is slowing as time goes by, thus the length of the day is increasing. This is due to mostly to the gravitational effects of the moon with rotational energy and angular momentum being transferred to the moon. However other factors such as large scale events like the Indian Ocean earthquake in 2004 have caused rotation to speed up, while post glacial rebound is changing the distribution of the earth’s mass, thus also impacting rotational speed. So counting on the earth’s rotation to determine the length of the day is not reliable on its own.

This is why UTC (or Universal Coordinated Time) is determined by a combination of UT1 (Universal Time or the aforementioned variable speed of the earth’s rotation, also called Astronomical Time), and TAI (International Atomic Time). TAI is determined by combining the output of over 200 atomic clocks in over 50 laboratories around the world. The atomic clocks use the time it takes a Cesium-133 atom at ground state to oscillate exactly 9,192,631,770 times to define the length of a second. TAI is constantly compared to UT1. Before the difference between the two scales reaches 0.9 seconds, a leap second is added to UTC.

This “leap second” ensures that astronomical time doesn’t get too out of sync with atomic time, however these manually added seconds can cause problems for technical devices, such as GPS and stock market programs, potentially resulting in costly crashes. Since 1972 25 leap seconds have been added and the next one will be added on June 30 at 23:59:50 to make the time 23:59:60 before the clock ticks over to July 1 00:00:00.

So what does this mean to you, me, IBM i and iCluster? Not a lot it turns out. IBM is stating that the extra second will not impact IBM i http://www.rpgpgm.com/2015/03/leap-second-coming-at-end-of-june.html , thus journaling will not break and iCluster will be fine. Note however that unless your IBM i OS is configured to use NTP time protocol service via the network, the extra second will NOT be automatically added to your system clock and you will need to change the QTIME system value.

So enjoy the extra second of sleep on June 30th 2015!

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  • Steve Allen Reply

    March 31, 2015 at 2:37 pm

    What is the meaning of “counting on the earth’s rotation to determine the length of the day is not reliable on its own”? If the day is not defined by the rotation of the earth, then what is the meaning of a day and how is it measured?

  • Jason Johnson Reply

    April 1, 2015 at 12:27 pm

    A day is defined by the combination of the rotation of the Earth and an integer number of seconds (usually 86,400, but not required). I believe that statement means that using the earth’s rotation alone would make computing and physics unreliable.

  • Anthony Youngman Reply

    April 7, 2015 at 9:02 pm


    The thing to remember is that a day (as defined as “the time between two middays”) IS NOT CONSTANT. Leap seconds were only introduced in 1972 because before that we couldn’t measure accurately enough. Bit like when we switched from the Julian calendar to the Gregorian because we realised our definition of a year as being 365 1/4 days was a bit off…

    And because we can be confident (because the physics says so) that the general trend is towards days getting slowly longer, we only ever add leap seconds. Should some unexpected event shorten the day slightly, we know it will self-correct. The day will only stop lengthening when we reach equilibrium with the sun and 1 day == 1 year. This is already the case for Mercury, and not far off for Venus (1 venus year == 2 venus days).

    So we need leap seconds to keep our atomic and solar days in sync – otherwise we could end up with atomic midday occurring at solar midnight! (Like Julian midwinter solstice now occurs in the middle of January).


  • Mike Warkentin Reply

    April 26, 2015 at 8:30 pm

    Excellent background information everyone! Thanks.

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