Syntax:
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.
