Jack Forster, writing for Hodinkee:
There probably were not a lot of us that noticed it during the countdown to midnight, and the New Year, last Saturday, but this year, time needed a tweak. At 23:59:59 on December 31, 2016, an additional second was added to UTC (Universal Time Coordinated, the international time standard) so that, for exactly one second, UTC time was 23:59:60.
This might sound a little ‘who cares’ for most of us, but managing the Leap Second is, among other things, essential for little things like running the Internet, and ensuring GPS doesn’t think you’re halfway to the Moon when you’re just trying to find your mother-in-law’s house (literally).
Accurate time is also essential for things like HTTPS certificates and, apparently, Cloudfare’s CDN services.