Java – localdate minus period gets the wrong result

Subtract a period from localdate (such as "28 years, 1 month and 27 days") to get the wrong result

But subtracting a period (only days, such as "10282" days) gives the correct result Is there anything to note?

public static void main(String[] args) {
    printAgeAndBirthday(1989,2,22);

    printBirthdayFromPeriod(28,1,27);
}
  private static void printBirthdayFromPeriod(int years,int months,int days) {
    final Period period = Period.of(years,months,days);
    final LocalDate Now = LocalDate.Now();
    final LocalDate birthday = Now.minus(28,ChronoUnit.YEARS)
            .minus(1,ChronoUnit.MONTHS)
            .minus(27,ChronoUnit.DAYS);

    System.out.println("your birthday is : "+ birthday);//1989-02-19
    System.out.println("your birthday is : "+ Now.minusYears(28).minusMonths(1).minusDays(27));//1989-02-19
    System.out.println("your birthday is : "+ Now.minus(period));//1989-02-19
    System.out.println("your birthday is : "+period.subtractFrom(Now));//1989-02-19
    System.out.println("your birthday is : "+ Now.minus(Period.ofDays(10282)));//1989-02-22
}

private static void printAgeAndBirthday(int year,int month,int dayOfMonth) {
    LocalDate today = LocalDate.Now();
    LocalDate birthday = LocalDate.of(year,month,dayOfMonth);

    Period p = Period.between(birthday,today);
    long p2 = ChronoUnit.DAYS.between(birthday,today);
    System.out.printf("You are %d years,%d months,and %d days old. (%d days total)%n",p.getYears(),p.getMonths(),p.getDays(),p2);

    LocalDate nextBDay = birthday.withYear(today.getYear());

    //If your birthday has occurred this year already,add 1 to the year.
    if (nextBDay.isBefore(today) || nextBDay.isEqual(today)) {
        nextBDay = nextBDay.plusYears(1);
    }

    Period p_1 = Period.between(today,nextBDay);
    long p_2 = ChronoUnit.DAYS.between(today,nextBDay);
    System.out.printf("There are %d months,and %d days until your next birthday. (%d total)%n",p_1.getMonths(),p_1.getDays(),p_2);
}

Console log:

You are 28 years,1 months,and 27 days old. (10282 days total)
There are 10 months,and 4 days until your next birthday. (310 total)
your birthday is : 1989-02-19
your birthday is : 1989-02-19
your birthday is : 1989-02-19
your birthday is : 1989-02-19
your birthday is : 1989-02-22

Java version: jdk1.0 8.0_ forty-five

Solution

Your case can be simplified to

LocalDate date1 = LocalDate.of(2017,22),date2 = LocalDate.of(2017,4,18);
Period p = Period.between(date1,date2);
System.out.println("date1 + p: "+date1.plus(p));
System.out.println("date2 - p: "+date2.minus(p));

This will print

date1 + p: 2017-04-18
date2 - p: 2017-02-19

In other words, the number of years is irrelevant (unless one is a leap year and the other is not, but here neither is) The following illustrates the problem:

February                       March                                                                                        April
19 20 21 22 23 24 25 26 27 28  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18
 ↑        │                                                                                   ↑                                                                                ↑
 │        └──────────────────────────── plus one Month ───────────────────────────────────────┴───────────────────────── plus 27 days ─────────────────────────────────────────┤
 │                                                                                ↑                                                                                            ↓
 └───────────────────────── minus 27 days ────────────────────────────────────────┴─────────────────── minus one month ────────────────────────────────────────────────────────┘

If you change direction, this will change:

Period p2 = Period.between(date2,date1);
System.out.println("date1 - p2: "+date1.minus(p2));
System.out.println("date2 + p2: "+date2.plus(p2));

This will print

date1 - p2: 2017-04-15
date2 + p2: 2017-02-22

Therefore, when you represent periods in years, months, and days, directions become relevant On the contrary, the average number of days between two dates is constant:

LocalDate date1 = LocalDate.of(2017,18);
Period p = Period.ofDays((int)ChronoUnit.DAYS.between(date1,date2));
System.out.println("date1 + p: "+date1.plus(p));
System.out.println("date2 - p: "+date2.minus(p));

date1 + p: 2017-04-18
date2 - p: 2017-02-22