   DATE_ADD(date,INTERVAL expr type)
   DATE_SUB(date,INTERVAL expr type)

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.  type is a keyword indicating
how the expression should be interpreted.
