ADD_MONTHS returns a new date_expression or timestamp_expression with YEAR and MONTH fields adjusted to provide a correct date, but a DAY field adjusted only to guarantee a valid date, which may not be a date you expect.
If this behavior is not acceptable for your application, use ANSI SQL:2011 DateTime interval arithmetic instead. See ANSI Interval Expressions.
ADD_MONTHS changes the DAY value of the result only to prevent reporting a nonvalid date_expression or timestamp_expression.