on Windows (with Visual Studio), mktime() also sets tm_wday only if 
successful.

But negative time_t are still not allowed by the Microsoft CRT, the 
tests fail.
There are workaround to this - for example python could use techniques 
similar to http://robertinventor.com/software/t64/
 
OTOH, the docs of the time module explicitly says that dates before the 
Epoch are not handled. Do you want to change this? in other words: is 
this a bug or a feature request?
http://docs.python.org/library/time.html