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

These functions perform date arithmetic. The date argument specifies
the starting date or datetime value. 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/5.1/en/date-and-time-functions.html

