P1 and P2 should be in the format YYMM or YYYYMM.
The period arguments P1 and P2 are not date values.
mysql>
mysql> SELECT PERIOD_DIFF(9803,199803);
+--------------------------+
| PERIOD_DIFF(9803,199803) |
+--------------------------+
| 0 |
+--------------------------+
1 row in set (0.00 sec)
mysql>