I was recently asked how syslog handles leap seconds. I thought it would be useful to reproduce my thoughts, initially expressed via private mail, here in the blog.
RFC5424 specifically forbids leap seconds, as during our discussions we found many cases where leap seconds caused grief. I also think the the TAI is considering aborting the use of leap seconds for this reason as well. To the best of my knowledge, GPS also does not use leap seconds. The ultimate reason to abandon UTC leap seconds in syslog was the we failed to identify an operating system that would expose leap seconds to a user process. So a syslogd or any other syslog sender would not even be able to see that one was introduced. From the syslog perspective, a leap second is just like any other second, but time flows "somewhat slower". I guess we are in the same boat as many operating systems with this perspective.
In RFC5424 we didn't explicitly state what time stamp should be written during a leap second - because we thought it could actually never happen (why? explained above!). But I would say that "Leap seconds MUST NOT be used" to me means that it should be expressed as the 59th second of said minute. But even if you bump the minute and use the 0 second, I cannot see how this should be problematic. On a single system, time should still evolve serially. For correlating events form multiple systems, the timestamp alone is insufficient in any case. You cannot closely enough synchronize the different real time clocks. So you need a different meachanism (like Lamport clocks) for this in any case.