Use mysql date_add() which will do the trick.
mysql> select date_add('2014-05-23T13:10:01.515', INTERVAL 2 month);
+-------------------------------------------------------+
| date_add('2014-05-23T13:10:01.515', INTERVAL 2 month) |
+-------------------------------------------------------+
| 2014-07-23 13:10:01.515000 |
+-------------------------------------------------------+
So in the query just add
date_add(BU_START_DATE, INTERVAL BU_DURATION month) as END_DATE