Did you ever watch what happens with a
sleep during a leap second or what date will show us? Most UNIX systems can handle a sleep with a fraction of a second and the date command can display milliseconds. I didn't look at the source but will the sleep wake up during the leap second? probably yes. And what will "date" show? the POSIX definition says it is 23:59:60
If I have time I will take a close look at this script:
while true; do sleep 0.1; date '+%m%d %H:%M:%S %3N'; done
The leap-seconds.3629404800 file says 1 Jul 2015 which I installed for the ntp daemon.
Then we have the next possibility to watch.