This article describes how the Windows Time service treats a leap second.
When the Windows Time service is working as a Network Time Protocol (NTP) client
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.
When the Windows Time service is working as an NTP server
No method exists to include a leap second explicitly for the Windows Time service when the service is working as an NTP server.
However, if an external NTP server sends a Leap Indicator that has a value of 01 to the Windows Time service NTP server, the Windows NTP server sends the same value to following NTP client.
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