[time-nuts] windows and leap seconds

[Jim Lux]

https://support.microsoft.com/en-us/kb/909614

basically: ignore the leap second, so your computer is "fast" relative 
to UTC, and next time you resynchronize time it adjusts

Windows, these days, runs NTP

http://blogs.msdn.com/b/mthree/archive/2015/01/08/leap-seconds-010815.aspx
http://blogs.msdn.com/b/mthree/archive/2015/01/14/leap-seconds-011415.aspx

for more info