fixed date add and subtract functions for dynamic values

lib/Doctrine/DBAL/Platforms/PostgreSqlPlatform.php

@@ -99,22 +99,22 @@ class PostgreSqlPlatform extends AbstractPlatform
 
     public function getDateAddDaysExpression($date, $days)
     {
-        return "(" . $date . "+ interval '" . $days . " day')";
+        return "($date + ($days || ' day')::interval)";
     }
 
     public function getDateSubDaysExpression($date, $days)
     {
-        return "(" . $date . "- interval '" . $days . " day')";
+        return "($date - ($days || ' day')::interval)";
     }
 
     public function getDateAddMonthExpression($date, $months)
     {
-        return "(" . $date . "+ interval '" . $months . " month')";
+        return "($date + ($months || ' month')::interval)";
     }
 
     public function getDateSubMonthExpression($date, $months)
     {
-        return "(" . $date . "- interval '" . $months . " month')";
+        return "($date - ($months || ' month')::interval)";
     }
     
     /**