Leap Seconds in OS X A leap second was recently added on June 30, 2015 23:59:60 UTC.

How are leap seconds implemented on OS X (specifically Yosemite)?
OS X and iOS use ntp-like software daemons to poll for and periodically adjust internal clocks based on external time sources.

A leap second arrives as an external correction similar to how each hardware clock corrects minor (or major) skews and inaccuracies periodically.

OS X ships with perl modules to accurately determine if a leap second is coming or has come, but internally it's not handled like leap years where the system knows in advance and automatically adjusts it's internal clock without needing to be corrected from an external time keeping source.


https://developer.apple.com/search/?q=leap%20second
Perl module LeapSecond 
time2posix man page