mktime() failure vs 31-DEC-1969 23:59:59 GMT

[Bradley White]

>                  The standards for mktime() define a return value of
> (time_t)-1 as indicating that the time specified cannot be
> represented.  However, a return value of -1 is also a valid time_t
> corresponding to 31-DEC-1969 23:59:59 GMT.  How do we distinguish
> between the two?

Call localtime() on the seemingly-in-error mktime() result, and compare
the relevant portions of the structs tm.  A match implies a non-error.