The normalization process is explained more formally as follows.
When the DAY field of the source date_expression or timestamp_expression is greater than the resulting target DAY field, ADD_MONTHS sets DD equal to the last day of the month + n to normalize the reported date or timestamp.
Define date_expression as 'YYYY-MM-DD' for simplicity.
For a given date_expression, you can then express the syntax of ADD_MONTHS as follows.
ADD_MONTHS('YYYY-MM-DD' , n)
Recalling that n can be negative, and substituting 'YYYY-MM-DD' for date_expression, you can redefine ADD_MONTHS in terms of ANSI SQL:2011 dates and intervals as follows.
ADD_MONTHS('YYYY-MM-DD', n) = 'YYYY-MM-DD' ± INTERVAL 'n' MONTH
The equation is true unless a non valid date such as 1999-09-31 results, in which case the ANSI expression traps the nonvalid date exception and returns an error.
ADD_MONTHS processes the exception and returns a valid, though not necessarily expected, date. The algorithm ADD_MONTHS uses to produce its normalized result is as follows, expressed as pseudocode.
WHEN DD > last_day_of_the_month(MM+n) THEN SET DD = last_day_of_the_month(MM+n)
This property is also true for the date portion of any timestamp_expression.
Normalization produces valid results for leap years.