A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time.
Since the leap second was introduced 25 adjustments has been made. On June 30, 2015 at 23:59:60 UTC there will be inserted a leap second again. This caused some issues on different Linux distributions back in 2012 when it happened the last time.
The leap second issue is not an application issue but a Linux server issue with servers using ntp. If you are running on RedHat everything related to the issue including work arounds is explained in this article on RedHat's site: https://access.redhat.
Work arounds should also be usable on Debian and other distros.