Microsoft plans to officially support "leap seconds" in Windows 10 and Windows Server editions, the company has revealed today.

Leap seconds are one-second adjustments made to Coordinated Universal Time (UTC) in order to keep Earth time in sync with solar time.

The reason why leap seconds appear is because of irregularities in the Earth's rate of rotation. Leap seconds have been accounted for since 1972, and they typically happen every 18 months.

When they do happen, one second is added at the end of the last minute of the day the leap second happens, or one second is subtracted when the leap second is negative (has never happened).

No "time smearing"

Microsoft and most software engineers have known about leap seconds for many years, but have not bothered to support them, with Linux creator Linus Torvalds once telling people to chill out about "leap seconds."

But as software, and especially Windows, became part of our daily lives and our critical infrastructure, US and EU regulators have issued orders that any software that handles critical tasks must be able to synchronize to UTC time with a very low margin of error, so it can execute critical tasks in sync with other equipment.

In order to comply with this requirement and improve Windows' time accuracy, Microsoft is updating upcoming versions of Windows 10 and Windows Server 2019 to support for the missing leap seconds.

According to a GIF published by Microsoft showing a Windows Server 2019 Insider Build, this is how a leap second will be added. Notice how the clock goes from 59 seconds to 60, and only then goes back to the normal 00.

Leap second addition

This is the standard way of adding a leap second. Microsoft said it was not going to support the practice of "time smearing," which consists of adding small time additions to standard time across days to account for the leap second without adding a full second at once.

PTP support coming to replace old NTP

Leap seconds support is currently being added to Windows 10 Redstone 5 and Windows Server 2019.

Leap second support is part of a larger set of new time-related features that Microsoft announced for Windows Server 2019 in a blog post published today.

Other new time-related features include more accurate software timestamping, better performance timers, and support for the new Precision Time Protocol (PTP), a replacement for the old Network Time Protocol (NTP).

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