On June 30, 2015, the world will once again experience a “leap second,” where one second is added to the clock.  As a result, there will actually be civilian clocks reading 23:59:60 on that day.  (Leap second has no impact on atomic clocks and anything that uses atomic clocks, like GPS.)

How could the addition of one second adversely affect your brand?  It can if you’re running a sweepstakes or contest where the computer acting as the timekeeper cannot handle 23:59:60 as a time setting.  Frequently, this means computers running UNIX or LINUX, but others can be affected as well.  In the past, the addition of a leap second has reportedly knocked systems and web sites (including Reddit) offline for hours.

For online contests and sweepstakes, it is common to accept entries up until a very specific date and time, such as 11:59:59 on June 30, 2015 (Eastern time).  Keep in mind that the extra second will be added as 23:59:60 UTC—that’s 8 PM Eastern time and 5 PM Pacific.  To address the leap second in 2015, the parent of the New York Stock Exchange has announced that it will cease trading (“delay market state transitions”) for 61 minutes between 23:00 UTC and 00:01 UTC (7 PM to 8 PM Eastern).

The reason behind the leap second is that the Earth’s rotation is slowing very, very slightly.  In order to help harmonize civilian and atomic clocks, the leap second is added to the civilian clock.  (The Time Service Department of the US Naval Observatory pointed out that “it is not possible to alter the Earth’s rotation speed to match the atomic clocks!”)  The leap second occurs only once every three or four years:  the preceding leap second was added in June of 2012; the leap second before that was added in December of 2008.

In the meantime, check with the IT Department of the timekeeper for your sweepstakes or contest to see whether leap second has already been addressed or whether you need to take any action.