There's a leap second coming up.

There’s a leap second coming up. Here’s a story of woe from the previous one, in 2012, from a sysop at fastmail, who had several redundant systems in several worldwide datacentres, failing apparently at random.
via the discussion at
which tells us that Amazon are going to skew the extra second in over a period of 24 hours - not totally unlike Google’s approach, although I don’t think they’ve said what their skew window is. Either way, times will be adrift by a half-second just fore and aft of the leap.
From the article:
“with very little work, even while not totally awake, the entire impact on our users was an ~15 minute outage for the few users who were primary on imap3 (less than 5% of our userbase) – plus delays of 5-10 minutes on incoming email to 16.7% of users each time a compute server crashed. While we didn’t do quite as well as Google, we did OK!”

Edit: FTR, there’s a link to
which describes UTC-SLS, a client-side slew in the final 1000 seconds before a leap, which allows time to be correct on every hour and half-hour but still monotonic. The document notes that Google’s approach is server-side: the NTP servers never announce a leap second and instead distribute a slewed version of time. Whereas Amazon slew over 24 hours, Google have not said exactly how they do it.