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. Not only is a day not exactly 24 hours (86,400 seconds) long, but it's also getting longer at about 2 milliseconds per century due to drag. Today, 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.

The next major update to Windows 10, likely due in October, and the next major version of Windows Server, named Windows Server 2019, will both include support for leap seconds. Whenever UTC needs an extra second to catch up, the clock in Windows will include the extra 61st second before rolling over to the next minute.

Technically, there can also be negative leap seconds, should UTC ever get more than 0.9 seconds ahead of atomic time. Windows will handle those, too, with the time jumping from :58 to :00 and skipping the 60th second. This is unlikely to ever be used, since there has never been a negative leap second. But rest assured, they'd work if there was one.

This work has been done to improve Windows' conformance to certain government regulations. Some regulations require the system clock to be accurate to within 100µs of an authoritative time source, and the system must track how it synchronizes its internal clock to the authoritative source. To address this need, the next Windows will include more logging and auditing capabilities for its clock management, as well as support for a new clock synchronization protocol called Precision Time Protocol (PTP). PTP replaces the venerable Network Time Protocol (NTP) that has long been used for syncing time between systems. The big thing that PTP adds is measurement of the latency of all the network hops between the clock source and the endpoint, which in turn enables greater accuracy for that endpoint. NTP will remain the default, as most people won't need the extra precision, but PTP will now be an option.