Also use relative date when comparing durations when unit is month

Author: catamorphism

polyfill/lib/ecmascript.mjs

@@ -3251,6 +3251,17 @@ function NudgeToCalendarUnit(
       r2 = months + increment * sign;
       startDuration = AdjustDateDurationRecord(duration.date, 0, 0, r1);
       endDuration = AdjustDateDurationRecord(duration.date, 0, 0, r2);
+      var cmpResult = compare(CombineDateAndTimeDuration(endDuration, TimeDuration.ZERO),
+                              duration);
+      if ((sign > 0 && cmpResult != 1) || (sign < 0 && cmpResult != -1)) {
+        r1 = r2;
+        r2 = months + (increment * 2) * sign;
+        endDuration = AdjustDateDurationRecord(duration.date, 0, 0, r2);
+        cmpResult = compare(CombineDateAndTimeDuration(endDuration, TimeDuration.ZERO), duration);
+        assert((sign > 0 && cmpResult == 1) || (sign < 0 && cmpResult == -1),
+               "nudgeToCalendarUnit: couldn't find larger duration");
+        startDuration = AdjustDateDurationRecord(duration.date, 0, 0, r1);
+      }
       break;
     }
     case 'week': {