Of course, it gets messy with having, for example, the last second of the year twice and code that cares about the encore second has to work hard to deal with it.   The consequences have been quite far reaching from that original decision: it's one of the reasons clock_gettime() has clocks for finding out whether it's time for lunch and clocks for determining how long lunch actually lasted.   Less prosaically, it's why at least one program that used getttimeofday() as an interval timer panicked when an sub-second interval not so much short as negative.  (The bug is now fixed, but the hideous workarounds are still being employed because no one is sure if it is actually fixed and no one wants to find out one dark and stormy night.)