ADD_MONTHS returns a date after adding the specified number of months to the specified date_string. The date will be returned in the default format (DELIM_M_D_Y) or the format specified by the optional input_format argument. If you want a different format as the output, use DATE_FORMAT.
ADD_MONTHS(12, '06-25-11') adds one year (12 months) to the specified date and returns the date 06-25-12.
ADD_MONTHS(3, '2011-06-25', DT_DELIM_Y_M_D) adds three months to the specified date and returns the date 2011-09-25.
ADD_MONTHS(1, '02-28-2011') returns the date 03-28-2011.
ADD_MONTHS(1, '03-31-2012') returns the date 04-30-2012.