Bug 798547 - Calculated Due Date is short 1 day when posting on...

day of fall change from Daylight Time to Standard Time.

Really any day where the period crosses the dst->std change, and
since the date dialog returns local midnight for the time that includes
the day of the change.

Convert both the parameter time and the return time to neutral time
to ensure that the interval is handled correctly.

libgnucash/engine/gncBillTerm.c

@@ -800,20 +800,21 @@ compute_monthyear (const GncBillTerm *term, time64 post_date,
 static time64
 compute_time (const GncBillTerm *term, time64 post_date, int days)
 {
-    time64 res = post_date;
+    time64 res = gnc_time64_get_day_neutral (post_date);
     int day, month, year;
 
     switch (term->type)
     {
     case GNC_TERM_TYPE_DAYS:
         res += (SECS_PER_DAY * days);
+        res = gnc_time64_get_day_neutral (res);
         break;
     case GNC_TERM_TYPE_PROXIMO:
         compute_monthyear (term, post_date, &month, &year);
         day = gnc_date_get_last_mday (month - 1, year);
         if (days < day)
             day = days;
-        res = gnc_dmy2time64 (day, month, year);
+        res = gnc_dmy2time64_neutral (day, month, year);
         break;
     }
     return res;