This article provides information on what Leap Second is.
Leap seconds have been added 26 times since 1972. They're inserted at the end of the last day of either June or December. On a leap year, leap second is added to official timekeeping on December 31, 2016.
2016-12-31 23.59.59 2016-12-31 23.59.60 <-- leap second 2017-01-01 00.00.00
Exinda's older kernel version has leap second issue. But newer version (v6, v7) has no issue with automatically inserting leap second.
Tested 6.4.0 and 7.0.3U1 with a script from the shell to see if leap seconds were inserted properly