Leap second on a Leap year

Sat Jun 30 2012 - Mel

Did you know that 1 minute is not always equivalent to 60 seconds?

On rare occasions, the world's standard time (UTC) is adjusted by adding or subtracting one second to a particular day, giving us a minute that has 61 or 59 seconds.  The last time a leap second was inserted was on December 31, 2008. The next one will be inserted just before Sunday, July 1 of this leap year.

If you are in London where local time is equal to UTC, the leap second will be inserted before midnight. Here in Japan, we are 9 hours advanced of UTC so it will be on Sunday at 8:59:60AM.

Why are leap seconds inserted anyway?

Like many relationship statuses in Facebook, the answer is "It's complicated". But let me try to make it uncomplicated for you.

Once upon a time...

The length of a second has been defined as "1 day ÷ (24hrs x 60mins x 60secs)" or 1/86,400th of the time it takes the earth to complete one rotation. Everyone was happy with this definition until scientists discovered that the average duration of the earth's rotation actually varies from year to year.

To get a more precise definition for the length of a second, scientists redefined the second in terms of the more stable period of the radiation of the Cesium atom. Since then, our time standard UTC has been based on this Atomic Time.

However, scientists also realized that the earth has been slowing down for the past years. This means that the average length of a day is getting longer. In other words, even when our atomic clock already says 12:00 noon, the sun wouldn't be exactly over our heads yet. To solve this problem without changing the definition of a second, there is a need to insert leap seconds to UTC.

So what?

A delay of one second isn't really a big deal if you're just like one of my FB friends who consistently post something like "June from today" on her FB status once the month changes. But if you're in the field of computing, you may have to pay attention. For instance, you would have to take account of leap seconds if you're trying to compute differences between two timestamps accurately. If you're logging events and transactions, you have to make sure that your system supports the inserted leap second.

Fortunately, the insertion of a leap second this time falls on a Sunday when the stock exchanges are closed. Companies involved in the financial industry (like the one which I work for) are relatively less likely to be affected by the leap second; nevertheless, our infrastructure team still prepared and made sure the leap second bug won't bite us.


"Is there not an appointed time to man upon earth? are not his days also like the days of an hireling?" - Job 7:1


Some companies have been affected by the leap second bug.