java.time.Period , dividing the period gives wrong results

Question

I tried using java.time.Period, and the results were different from my manual calculations by three days. The weird thing here is when I divide the period into two periods, the results matches my manual calculations.

The second method is just like how I calculate the period manually.

Is there something I have missed? Is there a standard method or algorithm of calendar arithmetic? And what's the algorithm used by java.time.Period ?

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

public class test2 {

    public static void main(String[] args) {

        LocalDate d1 = LocalDate.of(2014, 2, 14);
        LocalDate d2 = LocalDate.of(2017, 8, 1);

        Period p = Period.between(d1, d2);

        //using period between the two dates directly
        System.out.println("period between " + d1.toString() + " and " + d2.toString() + " is " + p.getYears()
                + " years " + p.getMonths() + " months " + p.getDays() + " Days");

        //dividing the period into two parts 
        p = Period.between(LocalDate.of(2014, 3, 1), d2);

        System.out
                .println("period between " + d1.toString() + " and " + d2.toString() + " is " + p.getYears() + " years "
                        + p.getMonths() + " months " + d1.until(LocalDate.of(2014, 3, 1), ChronoUnit.DAYS) + " Days");

    }
}

Answer 1

You perform two different operations here:

Period p = Period.between(d1, d2)

gives you a nicely formated way to output the difference between these dates (you have used the formatting options correctly).

d1.until(d2, ChronoUnit.DAYS) 

will give you the same - but not so nicely formated (basically it just gives you the number of days between the LocalDates).

Answer 2

You cannot divide the period when computing days difference as you did, because period computation depends on the month/ year of the related dates. In your example,

The period between 14 Feb and 1 March 2014 is just 15 days,

while the days period between 14 Feb and 1 Aug 2017 is computed from 14 Jul 2017 till 1 Aug 2017 which will be 18 days, in details:

1) 14/02/2014 -> 14/02/2017: 3 years
2) 14/02/2017 -> 14/07/2017: 5 months
3) 14/07/2017 -> 01/08/2017: 18 days

This means that replacing August 2017 by March 2014 to compute the days difference independently from the whole dates will lead to an inaccurate answer (15 while it should be 18 days).

Therefore, you may have to consider the whole dates if you're doing any manual computation and not just its day section, or, just simply and safely use Period class given methods to perform your required dates computations.

Answer 3

The answer (period between 2014-02-14 and 2017-08-01 is 3 years 5 months 18 Days) is what you should expect:

• it's 3 years from 2014-02-14 to 2017-02-14
• it's 5 months from 2017-02-14 to 2017-07-14
• it's 18 days from 2017-07-14 to 2017-08-01

---

The calculation proceeds from years to months to days. This allows you to calculate the number of years, months and days between 2014-02-14 and 2016-02-29, which gives 2 years and 15 days.

If you try to calculate the days first you have problems determining the number of years, months and days between 2014-02-14 and 2016-02-29, because there is no day 2014-02-29 - 14 days after 2014-02-14 its 2014-02-28, 15 days after 2014-02-14 its 2014-03-01.