The leap second is a scheduled one second adjustment to the system clock to ensure that system time remains synchronized with the sun. This is done by either adding an extra 23:59:60 second to the clock or skipping the 23:59:60 second on the selected day. Leap seconds do not occur on a strict schedule but are scheduled by the International Earth Rotation and Reference Systems Service (IERS) based on changes to the Earth's rotation.
Please see https://en.wikipedia.org/wiki/Leap_second for a list of recent and upcoming leap seconds.
With the exception of a minor issue in displaying log events that occurs during the leap second there are no known issues with the operation of Messaging Gateway (SMG and SMG-SP) products resulting from the insertion of a leap second. In most cases, internal date/time tracking is based on the epoch time rather than a date string generated by the operating system. Additionally, there were no reported issues with the Messaging Gateway following the insertion of recent leap seconds.
Systems using NTP time synchronization should automatically account for the leap second correction but customers who are concerned regarding interactions between the leap second and Messaging Gateway may disable NTP time synchronization prior to the addition of the leap second and re-enable time synchronization following the addition of the leap second to the time source.
Symantec recommends that where possible Messaging Gateway systems use NTP time synchronization.