Logged In: YES 
user_id=116933

I have a draft of the C99 standard, which in paragraph
7.23.2.3 describes mktime, and says:

"The original values of the tm_wday and tm_yday components
of the structure are ignored..."

So, I would say that mktime() is *not* underspecified by ISO C.

In order to write correct programs, it is neccessary to know
the semantics of the functions, and IMHO the manual should
specify what can be expected of them.