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 \

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