Syntax:
DATE_ADD(date,INTERVAL expr unit), DATE_SUB(date,INTERVAL expr unit)

These functions perform date arithmetic. date is a DATETIME or DATE
value specifying the starting date. expr is an expression specifying
the interval value to be added or subtracted from the starting date.
expr is a string; it may start with a `-' for negative intervals. unit
is a keyword indicating the units in which the expression should be
interpreted.

URL: http://dev.mysql.com/doc/refman/5.0/en/date-and-time-functions.html

