No, it seems that this year had not only an extra day, but also an extra second. Leap seconds occur when you have to factor in for drifting in the total amount of time per year over several decades or centuries. Basically, we aren't getting exactly 365 days to the second, we're getting slight variations each year due to all these factors, like the orbit of the Earth around the sun... it's complicated... but they add these little bits here and there to make the modern calender work.
I suppose a wikipedia link would be better.
I guess no one told MS about it this year. With all those people working on things, you think they would have had at least one person notice it.