Why would a Unix application ever see the :60? Any time someone checks the clock, the time should be derived from Unix time (seconds since the epoch) which doesn't account for leap seconds. So to an application it should appear as a duplicate :00 or :59.
localtime(3) tells me that
tm_sec The number of seconds after the minute, normally in the range
0 to 59, but can be up to 60 to allow for leap seconds.
The problem is that the epoch returned by time(2) defined by POSIX is idiotic:
POSIX.1 defines seconds since the Epoch as a value to be interpreted as
the number of seconds between a specified time and the Epoch, according
to a formula for conversion from UTC equivalent to conversion on the
naive basis that leap seconds are ignored and all years divisible by 4
are leap years. This value is not the same as the actual number of
seconds between the time and the Epoch, because of leap seconds and
because clocks are not required to be synchronized to a standard referâ
ence.
The correct thing to do would have been to define time() to return TAI or the like. Monotonic. Calculations involving deltas of time should just be deltas of that epoch value in seconds. Easy. If you find yourself trying to manipulate individual components of datetime stamps for purposes other than display and input, You're Doing Things Wrong. The only thing that would care about seconds being 60 would be the presentation layer. localtime() return tm_sec 60, so display 60. No need to "sanity check" (wrongly) the value returned by system libraries to be between 0 and 59 - they should be written by people smarter than yourself usually and they surely know about leap seconds, right? Right?
But unfortunately, idiocy was encoded into the standards, so further idiocy is required to work around the original idiocy. Before long, the world is being run by idiots. And it becomes even harder to undo the idiocy in higher layers even if you're competent and know what you are doing.
