Actually, the behavior of MySQL depends upon the allow_invalid_dates mode. If this mode is enabled then MySQL will accept the invalid date and it would perform the date arithmetic as it performs with a valid date. Otherwise, if this mode is inactive then it will not accept the invalid date and would produce NULL as output.
mysql> select '2017-02-30' + INTERVAL 7 day; +-------------------------------+ | '2017-02-30' + INTERVAL 7 day | +-------------------------------+ | NULL | +-------------------------------+ 1 row in set, 1 warning (0.00 sec)
Now after enabling the allow_invalid_date mode MySQL accepting the invalid dates and performing the date arithmetic as follows −
mysql> set sql_mode = allow_invalid_dates; Query OK, 0 rows affected (0.05 sec) mysql> select '2017-02-30' + INTERVAL 7 day; +-------------------------------+ | '2017-02-30' + INTERVAL 7 day | +-------------------------------+ | 2017-03-09 | +-------------------------------+ 1 row in set (0.00 sec) mysql> select '2017-11-31' + INTERVAL 7 day; +-------------------------------+ | '2017-11-31' + INTERVAL 7 day | +-------------------------------+ | 2017-12-08 | +-------------------------------+ 1 row in set (0.00 sec)