A leap second was recently added on June 30, 2015 23:59:60 UTC.
How are leap seconds implemented in Windows (specifically Windows 7)?
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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.
Windows is normally acting as an NTP client. It will just get its time up to date during the next update from the NTP server, which clearly has methods of handling leap seconds. This will happen, provided your system is setup to synchronize time via NTP. If it is not, then nothing will happen as Windows 7 does not have built-in functionality to deal with leap seconds
They are not implemented.
Just like a wristwatch, PC time-keeping is dumb. As others have mentioned, Windows 7 uses NTP to sync PC time to a known source. Which is the equivalent of you adjusting your wristwatch every now and then.
However, the leap-second does become a problem if you intend to write code that involves date/time math, where the number of seconds between two dates is important, AND where those calculations must match up with some kind of external system or data that is leap-second aware. I'm struggling to think of any such scenarios, but I imagine there are some out there.
Windows just keeps ticking along. There is no provision for leap seconds in Windows time service. It's also important to note that no major operating system has a provision for leap seconds.
At first glance, this may sound strange. If you understand what a leap second is for and when they are added, it becomes very simple.
The International Earth Rotation and Reference Systems Service monitors the earth's rotation for irregularities. Some factors alter the earth's rotation. These include tidal friction and the melting of continental ice sheets (weight changes!). In aggregate, the earth is slowing ever so slightly.
Leap seconds are irregularly spaced because the Earth's rotation speed changes irregularly. Indeed, the Earth's rotation is quite unpredictable in the long term, which explains why leap seconds are announced only six months in advance.
Since it is irregular and announced as needed, all modern operating systems rely on Network Time Protocol (NTP) to recover from the leap second. This is such a widespread and known issue with no graceful solution that asian markets delayed their opening by five minutes to give the computers enough time to check NTP for a time update during the last leap second.
Microsoft doesn't care about providing super accurate time for Windows clients. They cut corners and didn't handle special cases like leap seconds for desktop clients. Fortunately, NTP servers keep better time, therefore just keep syncing!
The W32Time service is not a full-featured NTP solution that meets time-sensitive application needs and is not supported by Microsoft as such.
As indicated in the Windows support article Support for the leap second:
Windows Server 2019 and Windows 10 October 2018 Update do support leap seconds in the platform.
The support article also explains how earlier versions on Windows do NOT support leap seconds. In particular:
Leap second processing is not handled separately by the Windows operating system (OS). For example, year, month, date, and time information in the following format is not supported by the Windows OS:
Therefore, 2012/7/1 08:59:60 is processed as 2012/7/1 09:00:00, per the ISO 8601 format.