Part two of the story around Leap Seconds and Windows: #NotY2K

Clip art from Microsoft Office OnlineWell, it’s been an interesting few days.

As I noted in my post on the story around Leap Seconds and Windows, the additional second in 2015 will not likely cause a Y2K, late-breaking change to daylight saving time or time zone boundaries. But that hasn’t stopped a lot of angst and hyperbole over the change coming this June.

Some reports (like this one in USA Today) were quick to associate the addition of a leap second in 2012 to the bug that “took down much of the Internet.” Not quite right as the timing of the leap second  coincided with a power outage that impacted their service provider (as noted by the BBC). Yes, there were some reported impacts as noted by Robert McMillan at Wired in his post on The Leap Second Is About to Rattle the Internet. But There’s a Plot to Kill It, calling out the underlying Linux operating system. Not one to throw stones over issues on time and date, as readers of my past posts will know, I have to call out one of the questions I’ve received more times than I can count this week:

“I hear there’s a new leap second coming. Do I have to update my Windows device? Will my device stop working? Will the Internet really melt down?”

The short answer is: generally*, no, as I’ve noted previously. As a user, you probably won’t notice the extra second nor see any impact to your Windows devices… nor will you see, I’ll wager, any impact to any of your other Internet connected devices regardless of the operating system it’s running.  As The Telegraph noted, "Many computing systems use the Network Time Protocol, or NTP, to keep themselves in sync with the world’s atomic clocks." And as you can Set the Clock on your Windows device, you can also sync your device’s clock with an Internet time server to help ensure your device’s clock is accurate:

“You can synchronize your computer clock with an Internet time server. This means that the clock on your computer is updated to match the clock on the time server, which can help ensure that the clock on your computer is accurate. Your clock is typically updated once a week and needs to be connected to the Internet for the synchronization to occur.

    1. Open Date and Time by clicking the Start button Picture of the Start button, clicking Control Panel, clicking Clock, Language, and Region, and then clicking Date and Time. [Note you can also do this by right-clicking on the clock on your desktop, or via the Search function and typing “Date and Time” in the box, or via the PC Settings -> Time and Language setting in Windows 8.]
    2. Click the Internet Time tab, and then click Change settings. Administrator permission required If you’re prompted for an administrator password or confirmation, type the password or provide confirmation.
    3. Select the check box next to Synchronize with an Internet time server, select a time server, and then click OK.

The Internet server is the default one and is maintained by Microsoft itself. The other four servers (e.g. are maintained by National Institute of Standards and Technology in the US.

(The important caveat here for people using a device at work (such as a PC tablet) is that computers on a domain (the network at work), you may not be able to synchronize your clock with an Internet time server as this is managed by your network administrator.)

As called out in MS KB article 909614

“The Windows Time service does not indicate the value of the Leap Indicator when the Windows Time service receives a packet that includes a leap second. (The Leap Indicator indicates whether an impending leap second is to be inserted or deleted in the last minute of the current day.) Therefore, after the leap second occurs, the NTP client that is running Windows Time service is one second faster than the actual time. This time difference is resolved at the next time synchronization. For more information about Windows time synchronization, go to the following Microsoft TechNet website, How the Windows Time Service Works

Basically, the clock in Windows  does not account for an additional leap second, as it’s dependent on the Network Time Protocol (aka NTP) – that’s networking protocol used to sync clocks of devices over the network. What I found interesting when working on DST back in 2006 was that it’s one of the original Internet protocols developed by Dr. David Mills (love his book, Computer Network Time Synchronization).

* Now, I said “generally” above when regarding general computer users and our customers running a Windows device wither at home or at work and accessing the Internet. Barring any impact to established Internet time servers (such has those presented in the sources offered in the Date and Time settings) and aren’t using a non-standard clock, most applications and services may be unaffected. Developers, sysadmins and IT professionals should know the leap second is not addressed in Windows until the next time sync following the official addition/ subtraction of the leap second, and that non Windows systems running on their networks may not account correctly for the leap second. These folks should also refer to the detailed post on high accuracy W32time requirements for more details on configuring the Windows Time service for high accuracy environments and Kerberos standards.

As well as Bob’s link to 10,000 Year Clock if you need even more accurate time measurement.

Also available via

Comments (0)

Skip to main content