"Leap Second": Witness the 61-seconds Minute on June 30th 2015

End of June 2015 (precisely on June 30th, 2015 at 23:59:59 UTC) an additional -Leap- second (23:59:60) will be added to UTC time thus affecting clock time around the world and everything else that comes with it.
If you are an IT personnel, you should check the impact of this event on your network by reading the next section below; else you can directly jump to "Leap Second, UTC, TAI and UT1" section where you will find a scientific explanation of what Leap second is all about.

What IT Personnel Need to Know:
Leap second addition is not an event that you want to overlook if you are an IT professional since it could cause problems on your network. The effect of Leap second addition can range from simple wrong clock display on certain nodes to high CPU/server hang/appliance crash in some other nodes. To know the potential problems your network might run into on the night of June 30th, the below vendors links must be cross-checked with the appliances and virtual machines running on your network:

If you would like to further check the impact of Leap second on your network, please open a case on Data Consult call center +961-1-511822.

Leap Second, UTC, TAI and UT1
You most likely already know about UTC (Coordinated Universal Time) which is basically the reference clock used worldwide, and from it offsets are added or removed, depending on Time Zone, in order to get the local clock of a region or country. What you probably didn't know is that UTC has 2 components:

International Atomic Time (TAI): based on extremely precise atomic clocks
Universal Time (UT1): a.k.a astronomical time, related to length of a day on Earth

Had we been living in a perfect world where the Earth's rotation speed is absolutely constant we would be following TAI to sync our clocks, but since it is not the case (due to Earth's rotation slowing down, see below for more information) then whenever the difference between TAI and UT1 reaches 0.9 seconds, a Leap second is added to UTC as restitution, ultimately updating all synched clocks around the world. This addition usually happens every few years, the closest upcoming one will occur on June 30th, 2015 at 23:59:59 UTC; while the last addition was made back in 2012.

Below is a graph showing the difference in seconds between UT1 and UTC over the years. Vertical segments correspond to leap seconds.

Why is Earth's rotation slowing down?
In brief, Earth's rotation is slowing down due to the moon's "attachment" to Earth via gravity. Without going too much into the technicalities, Paul Walorski gives a great example of how our Moon is affecting Earth's rotation:

"To picture what is happening, imagine yourself riding a bicycle on a track built around a Merry-go-Round. You are riding in the same direction that it is turning. If you have a lasso and rope one of the horses, you would gain speed and the Merry-Go-Round would lose some. In this analogy, you and your bike represent the Moon, the Merry-Go-Round is the rotating Earth, and your lasso is gravity. The slowing rotation of the Earth results in a longer day as well as a longer month. Once the length of a day equals the length of a month, the tidal friction mechanism will cease. (ie. Once your speed on the track matches the speed of the horses, you can't gain any more speed with your lasso trick.) That's been projected to happen once the day and month both equal about 47 (current) days, billions of years in the future." Source

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Elie Bassil
linkedin.com/in/eliebassil