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

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

mysql> SELECT DATE_SUB('1998-01-02', INTERVAL 31 DAY);
        -> '1997-12-02'
mysql> SELECT SUBDATE('1998-01-02', INTERVAL 31 DAY);
        -> '1997-12-02'

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 subtracted
from expr.

mysql> SELECT SUBDATE('1998-01-02 12:00:00', 31);
        -> '1997-12-02 12:00:00'
