Months between not returning correct value

Question

Select MOD (RUNC (MONTHS_BETWEEN (TO_DATE (\'28-Mar-2017\', \'DD-MON-YYYY\'),
 TO_DATE (\'28-Feb-2017\', \'DD-MON-YYYY\'))), 12) Months from dual

Why i

Answer 1

Because it is working to its documented specification:

MONTHS_BETWEEN returns number of months between dates date1 and date2. The month and the last day of the month are defined by the parameter NLS_CALENDAR. If date1 is later than date2, then the result is positive. If date1 is earlier than date2, then the result is negative. If date1 and date2 are either the same days of the month or both last days of months, then the result is always an integer. Otherwise Oracle Database calculates the fractional portion of the result based on a 31-day month and considers the difference in time components date1 and date2.

If you have requirements that are different to this then you will need to write your own function.

Answer 2

You are wrong. MONTHS_BETWEEN treats the time span from date '2017-02-28' to date '2017-03-28' as exactly one month (same day in two adjacent months). This can be read in the docs: https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions089.htm

MONTHS_BETWEEN returns number of months between dates date1 and date2. If date1 is later than date2, then the result is positive. If date1 is earlier than date2, then the result is negative. If date1 and date2 are either the same days of the month or both last days of months, then the result is always an integer. Otherwise Oracle Database calculates the fractional portion of the result based on a 31-day month and considers the difference in time components date1 and date2.

One date is bigger than the other so you get a positive or negative number (i.e. not zero) depending on which is first and which is second parameter, and as the days equal, the result will be an integer. That is 1 or -1 for adjacent months.

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Month calculation is a strange thing anyway, as a month is not a defined time span. You seem to have a certain definition in mind, which simply is different from how MONTHS_BETWEEN defines it. Nothing wrong about that. I would agree with MONTHS_BETWEEN in this case; from Feb 28 to Mar 28 is "exactly" one month. If you want different rules, then apply your own math :-)