Your dates are NOT native mysql dates (varchar, probably?), so you cannot use the mysql date functions on them directly. date('MM/DD/YY')
will not work reliably:
mysql> select month('03/03/14'), month('03/18/14');
+-------------------+-------------------+
| month('03/03/14') | month('03/18/14') |
+-------------------+-------------------+
| 3 | NULL |
+-------------------+-------------------+
Convert your date fields to native mysql date types, or take a huge hit on performance and convert them to native date values onthefly via str_to_date().