A Leap second is an extra second that is added to a day to keep UTC (Coordinated Universal Time) synchronized with "mean solar time". Leap seconds have been used 25 times since 1972 to stop the two drifting apart. The next Leap second will be added to June 30, 2015, and will mean that 23:59:60 UTC will be a valid time before changing to 00:00:00 UTC:
How does this effect the IBM i?
In an announcement released in January 14, 2015, IBM said:
The IBM i Operating System will not be affected by the Leap Second Adjustment. The IBM i Operating System will not automatically add the leap second at the end of June 30, 2015.
Which means that IBM i will not make the change for you.
In a later part of the announcement they said:
If your IBM i Operating System is configured to to utilize the NTP time protocol service via the network, the leap second will be adjusted by this service.
If your IBM i Operating System is not configured to utilize the NTP time protocol service via the network, you will need to make the leap second adjustment manually by changing the QTIME system value.
You can configure NTP on your IBM i using the 'Change SNTP attributes', CHGNTPA, command. This should be performed by a system administrator, and I am not going to give details on how to do this except to provide a link to IBM's documentation for this command at the bottom of this post.
If you do not use NTP then you will need to adjust the time manually on July 1, 2015.
You can read IBM's announcement in full here.