> But negative time_t are still not allowed by the Microsoft CRT, 
> the tests fail.
> (...)
> is this a bug or a feature request?

Linux mktime() supports any timestamp from 1901..2038. Should we limit 
the timestamp to 1970 just because of Microsoft? Test tm_wday fixes a 
bug on Linux and doesn't change the behaviour on Windows. So the 
problem is just the unit test: the test should be different on Windows 
(make sure that -1 raises an error).