Fix the 6-days-until-the-end-of-the-month bug

The addition causes the date to shift
forward into 1st of the next month, because a 0-based offset
is compared to be "more than" the days in the month instead of "more than
or equal to".

This is triggered by corner-cases where transition date is 6 days
off the end of the month and our calculations put it at N+1th day of the
month (where N is the number of days in the month). The subtraction should
be triggered to move the date back a week, putting it 6 days off the end;
for example, October 25 for CET DST transition; but due to incorrect comparison
the date isn't shifted back, we add 31 days to October 1st and end up
at November 1st).

Fixes issue #2215.

glib/gtimezone.c

@@ -1041,7 +1041,11 @@ find_relative_date (TimeZoneDate *buffer)
       /* week is 1 <= w <= 5, we need 0-based */
       days = 7 * (buffer->week - 1) + wday - first_wday;
 
-      while (days > days_in_month)
+      /* "days" is a 0-based offset from the 1st of the month.
+       * Adding days == days_in_month would bring us into the next month,
+       * hence the ">=" instead of just ">".
+       */
+      while (days >= days_in_month)
         days -= 7;
 
       g_date_add_days (&date, days);