LocalDate minus a Period get wrong result

Question

LocalDate minus a Period(like "28 years, 1 months and 27 days"),get wrong result.

But minus a Period(only have days unit ,like "10282"days) get right result. Is there anything to notice?

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);
}

the 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 : jdk1.8.0_45

Answer 1

Your case can be simplified to

LocalDate date1 = LocalDate.of(2017, 2, 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));

which will print

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

In other words, the number of years is irrelevant (unless one of the years involved is a leap year and the other isn’t, but here, both aren’t). The following illustrates the issue:

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 ────────────────────────────────────────────────────────┘

This will change, if you swap the direction:

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

which will print

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

So when you express a period in terms of years, month and days, the direction becomes relevant. In contrast, the plain number of days between two dates is invariant:

LocalDate date1 = LocalDate.of(2017, 2, 22), date2 = LocalDate.of(2017, 4, 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