A few vendors are reporting issues (even an unexpected reboot), be ready with a big mug of Coffee can be a long night! Many vendors are reporting to disable NTP before the adjustment.
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, or UT1. Without such a correction, time reckoned by Earth’s rotation drifts away from atomic time because of irregularities in the Earth’s rate of rotation. Since this system of correction was implemented in 1972, 26 such leap seconds have been inserted. The most recent one happened on June 30, 2012 at 23:59:60 UTC.A leap second will again be inserted at the end of June 30, 2015 at 23:59:60 UTC.