At least temporary fix for RT #127161

Which is really caused by RT #127168

src/core/Date.pm

@@ -83,7 +83,7 @@ my class Date does Dateish {
             self.new-from-daycount(self.daycount + $multiplier * $amount )
         }
         elsif $unit.starts-with('month') {
-            my int $month = $!month;
+            my Int $month = $!month;  # cannot be native because of RT #127168
             my int $year  = $!year;
             $month += $amount;
             $year += floor(($month - 1) / 12);