Windows: fix timezone calculation

Comparing to daylight-saving time change was performed incorrectly, so
that days in the same month as a change and the hour within the day before
the switch hour were all treated as pre-switch (instead of counting
only days up to the switch as pre-switch).

Thanks to Jon Zeppieri for the repair and George Neuner for
the report.

racket/src/racket/src/fun.c

@@ -9875,6 +9875,8 @@ static int is_day_before(SYSTEMTIME *a, SYSTEMTIME *b)
 
   if (a->wDay < doc)
     return 1;
+  if (a->wDay > doc)
+    return 0;
 
   dtxCOMP(wHour);
   dtxCOMP(wMinute);