Let me recap, just to make sure I have it straight.  There are two errors on Windows:

* The ! on (what is currently) line 3770 is wrong:
    if (!utime_read_time_arguments(&ua)) {

* If you pass in a Unicode string but also pass in both times and ns,
  you get the wrong error: effectively it's complaining that the string
  is not narrow, when it should be complaining about having both times
  and ns.

For the former, obviously removing the ! is correct.  But I like your idea of making the utime_read_time_argument() return value tell you what happened; that's what the Windows code needs to know.

So here it is!  Please see the attached patch.