Syntax:
ADDDATE(date,INTERVAL expr unit), ADDDATE(expr,days)

When invoked with the INTERVAL form of the second argument, ADDDATE()
is a synonym for DATE_ADD(). The related function SUBDATE() is a
synonym for DATE_SUB(). For information on the INTERVAL unit argument,
see the discussion for DATE_ADD().

MariaDB> SELECT DATE_ADD('2008-01-02', INTERVAL 31 DAY);
        -> '2008-02-02'
MariaDB> SELECT ADDDATE('2008-01-02', INTERVAL 31 DAY);
        -> '2008-02-02'

When invoked with the days form of the second argument, MySQL treats it
as an integer number of days to be added to expr.

URL: https://mariadb.com/kb/en/adddate/

