UNIX Seconds, TAI (SI) Seconds, Leap Seconds, and Real-World Code

A problem is that most documents do not use a vocabulary which can distinguish time scales without sentences of ambiguity. I suggest http://www.ucolick.org/~sla/leapsecs/picktwo.html as introduction to the problem that in historic use there are two unrelated kinds of seconds -- one which is a subdivision of a calendar day for residents of Earth, and one which is a constant duration as measured in a particular reference frame. Any time library which spans dates before and after 1970 is trying to make use of both kinds of second, and that ends up providing answers which are akin to a function that claims to provide arcsin(-2) -- which is to say that there are complexities involved that need careful explanations and definitions of exactly what is being considered important.