Java 8 calculate months between two dates

NOTE THIS IS NOT A DUPLICATE OF EITHER OF THE FOLLOWING

• Calculating the difference between two Java date instances
• calculate months between two dates in java [duplicate]

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I have two dates:

• Start date: "2016-08-31"
• End date: "2016-11-30"

Its 91 days duration between the above two dates, I expected my code to return 3 months duration, but the below methods only returned 2 months. Does anyone have a better suggestion? Or do you guys think this is a bug in Java 8? 91 days the duration only return 2 months.

Thank you very much for the help.

Method 1:

Period diff = Period.between(LocalDate.parse("2016-08-31"),
    LocalDate.parse("2016-11-30"));

Method 2:

long daysBetween = ChronoUnit.MONTHS.between(LocalDate.parse("2016-08-31"),
    LocalDate.parse("2016-11-30"));

Method 3:

I tried to use Joda library instead of Java 8 APIs, it works. it loos will return 3, It looks like Java duration months calculation also used days value. But in my case, i cannot use the Joda at my project. So still looking for other solutions.

	LocalDate dateBefore= LocalDate.parse("2016-08-31");
	LocalDate dateAfter = LocalDate.parse("2016-11-30");
	int months = Months.monthsBetween(dateBefore, dateAfter).getMonths();
	System.out.println(months);

Since you don't care about the days in your case. You only want the number of month between two dates, use the documentation of the period to adapt the dates, it used the days as explain by Jacob. Simply set the days of both instance to the same value (the first day of the month)

Period diff = Period.between(
			LocalDate.parse("2016-08-31").withDayOfMonth(1),
		    LocalDate.parse("2016-11-30").withDayOfMonth(1));
System.out.println(diff); //P3M

Same with the other solution :

long monthsBetween = ChronoUnit.MONTHS.between(
		LocalDate.parse("2016-08-31").withDayOfMonth(1),
	    LocalDate.parse("2016-11-30").withDayOfMonth(1));
System.out.println(monthsBetween); //3

Edit from @Olivier Grégoire comment:

Instead of using a LocalDate and set the day to the first of the month, we can use YearMonth that doesn't use the unit of days.

long monthsBetween = ChronoUnit.MONTHS.between(
     YearMonth.from(LocalDate.parse("2016-08-31")), 
     YearMonth.from(LocalDate.parse("2016-11-30"))
)
System.out.println(monthsBetween); //3

//Backward compatible with older Java

public static int monthsBetween(Date d1, Date d2){
    if(d2==null || d1==null){
        return -1;//Error
    }
    Calendar m_calendar=Calendar.getInstance();
    m_calendar.setTime(d1);
    int nMonth1=12*m_calendar.get(Calendar.YEAR)+m_calendar.get(Calendar.MONTH);
    m_calendar.setTime(d2);
    int nMonth2=12*m_calendar.get(Calendar.YEAR)+m_calendar.get(Calendar.MONTH);
    return java.lang.Math.abs(nMonth2-nMonth1);
}

Since Java8:

ChronoUnit.MONTHS.between(startDate, endDate);

The documentation of Period#between states the following:

>The start date is included, but the end date is not.

Furthermore:

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

Your end day-of-month 30 is not greater than or equal to your start day-of-month 31, so a third month is not considered.

Note the parameter names:

public static Period between​(LocalDate startDateInclusive, LocalDate endDateExclusive)

To return 3 months, you can increment the endDateExclusive by a single day.

In case you want stick to java.time.Period API

As per java.time.Period documentation

Period between(LocalDate startDateInclusive, LocalDate endDateExclusive)

where

@param startDateInclusive  the start date, inclusive, not null
@param endDateExclusive  the end date, exclusive, not null

So it is better to adjust your implementation to make your end date inclusive and get your desired result

Period diff = Period.between(LocalDate.parse("2016-08-31"),
                LocalDate.parse("2016-11-30").plusDays(1));
System.out.println("Months : " + diff.getMonths());
//Output -> Months : 3

You have to be careful, never use LocalDateTime to calculate months between two dates the result is weird and incorrect, always use LocalDate !

here's is some code to prove the above:

package stack.time;

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.temporal.ChronoUnit;

public class TestMonthsDateTime {
	public static void main(String[] args) {
		/**------------------Date Time----------------------------*/
		LocalDateTime t1 = LocalDateTime.now();
		LocalDateTime t2 = LocalDateTime.now().minusMonths(3);
		long dateTimeDiff = ChronoUnit.MONTHS.between(t2, t1);	
		System.out.println("diff dateTime : " + dateTimeDiff); // diff dateTime : 2
		/**-------------------------Date----------------------------*/
		LocalDate t3 = LocalDate.now();
		LocalDate t4 = LocalDate.now().minusMonths(3);
		long dateDiff = ChronoUnit.MONTHS.between(t4, t3);
		System.out.println("diff date : "  +  dateDiff); // diff date : 3
	}
}

My 2%

After the short investigation, still not totally fix my question, But I used a dirty solution to avoid return the incorrect duration. At least, we can get the reasonable duration months.

private static long durationMonths(LocalDate dateBefore, LocalDate dateAfter) {
		System.out.println(dateBefore+"   "+dateAfter);
		if (dateBefore.getDayOfMonth() > 28) {
			dateBefore = dateBefore.minusDays(5);
		} else if (dateAfter.getDayOfMonth() > 28) {
			dateAfter = dateAfter.minusDays(5);
		}
		return ChronoUnit.MONTHS.between(dateBefore, dateAfter);
	}

This example checks to see if the second date is the end of that month. If it is the end of that month and if the first date of month is greater than the second month date it will know it will need to add 1

LocalDate date1 = LocalDate.parse("2016-08-31");
LocalDate date2 = LocalDate.parse("2016-11-30");
	
long monthsBetween = ChronoUnit.MONTHS.between(
	        date1,
	        date2);
	
if (date1.isBefore(date2) 
		&& date2.getDayOfMonth() == date2.lengthOfMonth() 
		&& date1.getDayOfMonth() > date2.getDayOfMonth()) {
	monthsBetween += 1;
}