Я не вижу, что с этим не так:
mysql> select str_to_date('3/8/2010 12:31:00 AM', '%c/%e/%Y %k:%i:%s %p');
+-------------------------------------------------------------+
| str_to_date('3/8/2010 12:31:00 AM', '%c/%e/%Y %k:%i:%s %p') |
+-------------------------------------------------------------+
| NULL |
+-------------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> show warnings;
+-------+------+---------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+---------------------------------------------------------------------------+
| Error | 1411 | Incorrect datetime value: '3/8/2010 12:31:00 AM' for function str_to_date |
+-------+------+---------------------------------------------------------------------------+
1 row in set (0.00 sec)