MySQL treats the second argument as an integer number of days to be added to expr.
mysql> mysql> SELECT ADDDATE('1998-01-02', 31); +---------------------------+ | ADDDATE('1998-01-02', 31) | +---------------------------+ | 1998-02-02 | +---------------------------+ 1 row in set (0.00 sec) mysql>
14.2.ADDDATE | ||||
14.2.1. | ADDDATE(date,INTERVAL expr unit), | |||
14.2.2. | ADDDATE(expr,days) |