Date difference of months calculated wrong (other than expected)

[afiller]

My JUnit tests failed today at the last day of August because of the following problem:

They calculated the amout of months between now (2023-08-31) and in 3 months (2023-11-30) and came to the result of 2, which is in my opinion wrong:

2023-08-31 --> add 3 months (also Java comes to this result) --> 2023-11-30

...but Threeten-Extra means it's only 2:

        // Create dates for testing
        Date date1 = new Date(123, 7, 31, 12, 0, 0);
        Date date2 = new Date(123, 10, 30, 12, 0, 0);

        // Create ZoneDateTimes with the same zone
        ZonedDateTime dateTime1 = ZonedDateTime
                .from(date1.toInstant().atZone(ZoneId.systemDefault()));
        ZonedDateTime dateTime2 = ZonedDateTime
                .from(date2.toInstant().atZone(ZoneId.systemDefault()));

        // Show that dates are understood correctly
        System.err.println(date1);
        System.err.println(date2);
        // Calculate the months between
        System.err.println(Months.between(dateTime1, dateTime2).getAmount());

Output:

Thu Aug 31 12:00:00 CEST 2023
Thu Nov 30 12:00:00 CET 2023
2

It MAY come from the change from summer to winter time, but this should not have an effect on the months calculation.

Any ideas?

[perceptron8]

This is correct. See https://docs.oracle.com/javase/8/docs/api/java/time/Period.html#between-java.time.LocalDate-java.time.LocalDate-.

A month is considered if the end day-of-month is greater than or equal to the start day-of-month.

[jodastephen]

This is how java.time.* works, thus ThreeTen-Extra follows

[afiller]

Ok, thank you for clarifying.