Joda Time and Java8 Time difference

Question

I'm looking for a solution to calculate the months between two date. I think joda or java8 time can do it. But when I compare them I found something really weird.

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.temporal.ChronoUnit;
import java.util.Date;
import org.joda.time.Months;

public class DateMain {
    public static void main(String[] args) throws ParseException {

    SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
    Date d1 = simpleDateFormat.parse("2017-01-28 00:00:00.000");
    Date d2 = simpleDateFormat.parse("2017-02-28 00:00:00.000");

    System.out.println("Test Cast 1");
    System.out.println("joda time api result: " + monthsBetweenJoda(d1, d2) + " month");
    System.out.println("java8 time api result: " + monthsBetweenJava8(d1, d2) + " month");

    Date dd1 = simpleDateFormat.parse("2017-01-29 00:00:00.000");
    Date dd2 = simpleDateFormat.parse("2017-02-28 00:00:00.000");

    System.out.println("Test Cast 2");
    System.out.println("joda time api result: " + monthsBetweenJoda(dd1, dd2) + " month");
    System.out.println("java8 time api result: " + monthsBetweenJava8(dd1, dd2) + " month");
}

public static int monthsBetweenJoda(Date fromDate, Date toDate) {
    if (fromDate == null || toDate == null) {
        throw new IllegalArgumentException();
    }
    org.joda.time.LocalDateTime fromLocalDateTime = org.joda.time.LocalDateTime
        .fromDateFields(fromDate);
    org.joda.time.LocalDateTime toLocalDateTime = org.joda.time.LocalDateTime
        .fromDateFields(toDate);
    Months months = Months.monthsBetween(fromLocalDateTime, toLocalDateTime);
    return months.getMonths();
}

public static long monthsBetweenJava8(Date fromDate, Date toDate) {
    if (fromDate == null || toDate == null) {
        throw new IllegalArgumentException();
    }
    LocalDateTime ldt1 = fromDate.toInstant().atZone(ZoneId.systemDefault()).toLocalDateTime();
    LocalDateTime ldt2 = toDate.toInstant().atZone(ZoneId.systemDefault()).toLocalDateTime();
    return ChronoUnit.MONTHS.between(ldt1, ldt2);
}

}

the output is below:

Test Cast 1
joda time api result: 1 month
java8 time api result: 1 month
Test Cast 2
joda time api result: 1 month
java8 time api result: 0 month

I feel very confused with the test case 2, and which one is reasonable?

Sorry, it's my first question here. Completed code attached.

Answer 1

Joda-Time and Java 8 java.time.* have different algorithms for determining the number of months between.

java.time.* only declares a month has passed once the day-of-month is greater (or in the next month). From 2017-01-29 to 2017-02-28 it is clear that the second day-of-month (28) is less than the first (29) so a month has not yet been completed.

Joda-Time declares a month has passed because adding 1 month to 2017-01-29 will yield 2017-02-28.

Both are plausible algorithms, but I believe the java.time.* algorithm to be more in tune with what people expect (which is why I chose a different algorithm when writing java.time.*).