   ADDDATE(date,INTERVAL expr type)
   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 argument, see the
discussion for DATE_ADD().

mysql> SELECT DATE_ADD('1998-01-02', INTERVAL 31 DAY);
        -> '1998-02-02'
mysql> SELECT ADDDATE('1998-01-02', INTERVAL 31 DAY);
        -> '1998-02-02'

As of MySQL 4.1.1, the second syntax is allowed, where expr is a date
or datetime expression and days is the number of days to be added to
expr.

mysql> SELECT ADDDATE('1998-01-02', 31);
        -> '1998-02-02'
