Regarding Unix (POSIX) time, Wikipedia says:
Due to its handling of leap seconds, it is neither a linear representation of time nor a true representation of UTC.
$ date -d '@867715199' --utc
Mon Jun 30 23:59:59 UTC 1997
$ date -d '@867715200' --utc
Tue Jul 1 00:00:00 UTC 1997
Mon Jun 30 23:59:60 UTC 1997
The number of seconds per day are fixed with Unix timestamps.
The Unix time number is zero at the Unix epoch, and increases by exactly 86400 per day since the epoch.
So it cannot represent leap seconds. The OS will slow down the clock to accommodate for this. The leap seconds is simply not existent as far a Unix timestamps are concerned.