Date: 5 Dec 1999 01:22:15 -0000 From: "D. J. Bernstein" <djb@cr.yp.to> UTC includes leap seconds; the POSIX scale does not. This neglects the distinction between real-number counts of seconds (which are how these timescales are officially defined) and broken-down representations (which are meant for human consumption). If you're counting seconds, then (as I mentioned earlier) UTC omits completed leap seconds. This is the only way to make sense of the official definition of UTC in terms of an offset of seconds from TAI; see: http://hpiers.obspm.fr/webiers/general/earthor/utctable1.html If UTC second counts included leap seconds, there would be no reason to distinguish them from TAI second counts. Any system intended to supplant POSIX should be able to generate and accept UTC second counts, not only for backward compatibility, but also because it's often convenient to have them in their own right.