Windows To Better Support Leap Seconds

Windows 10’s next major update will add support for leap seconds. It’s designed to make the system compatible with some regulatory requirements

The leap second is necessary because of the way we keep track of time. Almost all timekeeping, including computer operations and the internet, is based on Coordinated Universal Time (UTC). That’s an atomic time signal based on the behaviour of caesium in labs.

The problem is that this doesn’t always perfectly match solar time, which is derived from the rotation of Earth. The two can drift apart because the rotation of Earth changes in an irregular manner. This isn’t, as sometimes thought, primarily to do with Earth moving away from the sun, but rather because of multiple factors including the moon’s tidal effects and changes in the mass distribution of Earth’s surface such as after major earthquakes.

If the disparity between UTC and solar time ever gets big enough, they are put back in sync (or at least closer to on another) via a leap second at the end of June or December. In theory this leap second could be negative if needed, but to date it’s always been solar time ‘slipping behind’ so an extra second is added to UTC before midnight.

This can be problematic with some computer systems either not getting the message about the change, being thrown by having a time signal that ‘shouldn’t exist’ or otherwise get into problems with a one-second disparity between systems. Some of the more common problems include GPS signals getting out of sync and financial trading records disagreeing about when a deal was made.

Until now, Windows has largely coped with leap seconds by relying on the idea that the computer will notice it’s own time was a second out of that shown by UTC reference signals. That’s been sufficient as Microsoft as somewhat amazingly worked on the basis that “the requirement for time accuracy on Windows was limited to domain-based scenarios that required all devices to be synchronized within 5 minutes.“

That’s no longer good enough as some government regulations require organizations to use systems and software where the time is accurate to a little as 100 microseconds (100 millionths of a second.)

To reach that goal, Microsoft is making several changes to the way it works with both time and leap seconds. One is to correctly apply the leap second in full at the correct moment, rather than use the previous tactic of “smearing” by which the system’s timekeeping is literally slowed down for a period. While that means avoiding having the troublesome “61st second”, it means the clock is inaccurate over a longer period.

Other changes include improving the way Windows system clocks take account of disparities caused by latency over networks, and increasing the frequency at which the clock’s timing is checked and corrected.

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