[jwm]

From the Wikipedia article on Leap Seconds:

> Because the Earth's rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS)

I don't see how applications could make unix second <-> day conversions without downloading a map from the IERS, if leap seconds were included.