My timezone is going to have an extra second added at 3:59:60 PM on December 31st, 2005. Fun; I wonder how many of the devices that I deal with will do the right thing with the extra second. Odds are most of them will just end up an extra second off. I assume that NTP has a way of dealing with this, although it might just be outside of the protocol’s scope–leap seconds really just change the seconds-since-some-epoch to human-visible-date mapping. (Update: it’s complicated)
Since leap seconds aren’t new, and I don’t really care about sub-second timing precision on any of my devices, I doubt I’ll even notice the change, although undoubtedly there are devices on the market that will have problems; I wouldn’t be surprised if there’s a cheap GPS receiver somewhere with leap seconds issues.
This reminds me of two of the pedantic sysadmin interview questions that I’ve never really had the guts to ask a real candidate–“exactly how many hours are in a day?” and “how many seconds are there in a minute?” Strictly speaking, the answers are “23, 24, or 25, depending on DST transitions” and “59, 60, or 61, depending on leap seconds.” The 23/24/25 thing actually bites new sysadmins–never schedule something that needs to happen exactly once per week to happen between 2:00 and 3:00 local time on a Sunday morning, because once per year it won’t happen at all, and another time it’ll happen twice.