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 difference is resolved at the next time synchronization.
When the Windows Time service is working as an NTP server
No method exists to include a leap second for the Windows Time service.
How a leap second is included depends on NTP server settings.