A leap second is a one-second adjustment that is applied to Coordinated Universal Time (UTC) in order to synchronize atomic clocks with astronomical clocks. The Earth's rotation around its own axis is slowing down gradually so a second is often added to compensate for this. Without this adjustment, there will be an increasing gap in the time between atomic time and astronomical time.
NOTE: The last leap second was added on December 31, 2016 at 23:59:60. The next possible Leap Second may be added on December 31, 2020.
The Palo Alto Networks firewall handles the leap second, as well as the leap year, in the following manner: