The brilliant thing about the smear is that it distributes the new second across each second of the day, so that each second differed by 1/86400 seconds, well within the margin of error for NTP.
As far as the computers were concerned, nothing was different.
The less brilliant thing about the smear is that if your ntpd syncs from smeared and unsmeared servers, the results aren't great.
It would have been better if they would have kept the time on the wire accurate or added mandatory protocol stuff to avoid confusing things for ntpds configured to different leap second handling.
Better still, if Unix time tracked TAI (monotonically increasing every SI second), and Leap Seconds were handled in a distributed database much like time zones - possibly even the same one we all use anyway.
if you need below 1 millisecond time accuracy, probably you know what you are doing and you wont mix NTP servers (and I think you need PTP for that)
Until you work in financial markets and you want to measure latency vs a bunch of other servers you don't own, all of whom pick different smears (and some of whom aren't sure in advance which smears).
financial markets dont trade when the smears happen (NYE)
Early in my career, I had to babysit our FX quoting app through the July 2015 leap second.
And some things do trade 24/7 (though I wish they wouldn't, one auction per minute during US east coast business hours would be totally sufficient for most US markets).
They can happen mid-year too. They usually don't for this reason, but they can.
Depends on the market. Crypto and prediction markets wont stop.
You'd think so. Legacy servers that've been running for ten years really puts a hitch in that.
User error