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

[Arthur David Olson]

> >                  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.

Since there's normalization going on in mktime, the tm that comes back from
localtime may not match the one that went in to mktime even if -1 is being
returned as a legitimate time_t rather than an error indication.

Another possibility is to save away the original tm structure; if mktime
returns -1, bump the tm_sec value by 1, call mktime again, and see if it
returns zero.

				--ado