Syntax:
TIMESTAMPADD(unit,interval,datetime_expr)

Adds the integer expression interval to the date or datetime expression
datetime_expr. The unit for interval is given by the unit argument,
which should be one of the following values: FRAC_SECOND
(microseconds), SECOND, MINUTE, HOUR, DAY, WEEK, MONTH, QUARTER, or
YEAR.

Beginning with MySQL 5.1.24, it is possible to use MICROSECOND in place
of FRAC_SECOND with this function, and FRAC_SECOND is deprecated.

The unit value may be specified using one of keywords as shown, or with
a prefix of SQL_TSI_. For example, DAY and SQL_TSI_DAY both are legal.

URL: http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html

