The GIF below illustrates how Windows will deal with leap seconds, which are added approximately every 18 months to synchronize clocks with the Earth's slowing rotation. Instead of jumping straight to the next second, Windows 10 will now display the leap second:
Right now, Windows 10 directly jumps from 16:59:59 to 17:00:00 without a leap second, but it will now include 16:59:60 when there is an actual leap second. That one extra second is quite important for time accuracy, especially with increased demand for higher accuracy time from government regulations, according to Microsoft. The company says it will not include an option for leap second smearing where the extra second would be split up into smaller units and added throughout the day as it will prevent the OS from meeting accuracy requirements.