https://arstechnica.com/gadgets/2018/07/windows-10-will-get-better-at-telling-the-time-with-new-leap-second-support/

One of the things that to me sums up the utter futility of existence and the inevitability of humanity's eventual extinction is the fact that the world is slowing down. About every 18 months or so, the Earth takes about a second longer to rotate on its axis, ever so slowly grinding to a halt. There's at least a possibility that when this happens, the Earth will be tidally locked to the Sun, with one side in sun-scorched perpetual daylight (probably Texas, they frankly won't notice the extra heat) and the other side eternally dark. The future is really bleak.

But between now and then, we have to handle the problem of keeping track of the time. There are two main sources of time; a whole bunch of atomic clocks averaged together to produce International Atomic Time, and the astronomical time that comes from measuring how long the earth actually takes to spin on its axis. This latter time, named UTC ("coordinated universal time"), is used in science and engineering. For most purposes, it's the time reference that we want our watches, clocks, phones, and computers to be set by. Because UTC is based on the Earth's actual spinning, it slowly falls behind atomic time. Every time the gap is more than 0.9 seconds, an extra second is added to UTC—a leap second—to bring the two back in sync.