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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