如何获取两个日期之间的月数?

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英文:

How to get number of months between dates?

问题

nownow + 2 个月 之间的差异在这个示例中不等于2,尽管我认为LocalDate的数学运算应该是这样的:

import java.time.LocalDate;
import static java.time.temporal.ChronoUnit.MONTHS;

public class MyClass {
    public static void main(String... args) {
        LocalDate now = LocalDate.of(2020, 7, 31);
        LocalDate later = now.plusMonths(2);
        
        System.out.println("Now: " + now);
        System.out.println("Later: " + later);
        System.out.println("Months between now and later: " + MONTHS.between(now, later));
    }
}

输出结果为:

Now: 2020-07-31
Later: 2020-09-30
Months between now and later: 1

我之所以发现这一点,是因为我碰巧运行了一个单元测试,该测试落在一个打破预期的日期上...

查看LocalDate.plusMonths的javadoc:

此方法将指定的月数添加到月份字段,分为三个步骤:

  1. 将输入的月份添加到年份的月份字段。
  2. 检查生成的日期是否无效。
  3. 必要时调整日期的日字段为最后一个有效日期。

例如,2007-03-31 加一月会导致无效日期 2007-04-31。而不是返回无效结果,选择了该月的最后一个有效日期 2007-04-30。

这意味着这是按照预期工作的。所以,不需要使用老式的 Date/Time API...

获取两个日期之间的月份数的正确方法是什么?

英文:

The difference between now and now + 2 months not equal to 2 in this example, despite my thinking LocalDate math worked this way:

import java.time.LocalDate;
import static java.time.temporal.ChronoUnit.MONTHS;

public class MyClass {
    public static void main(String... args) {
            LocalDate now = LocalDate.of(2020, 7, 31);
            LocalDate later = now.plusMonths(2);
            
            System.out.println("Now: " + now);
            System.out.println("Later: " + later);
            System.out.println("Months between now and later: " + MONTHS.between(now, later));
    }
}

Outputs:

Now: 2020-07-31
Later: 2020-09-30
Months between now and later: 1

I found this out only because I happened to run a unit test that fell on a date that breaks the expectation...

Reviewing the javadoc for LocalDate.addMonths:

> This method adds the specified amount to the months field in three
> steps:
>
> Add the input months to the month-of-year field
> Check if the resulting date would be invalid
> Adjust the day-of-month to the last valid day if necessary
>
> For example, 2007-03-31 plus one month would result in the invalid
> date 2007-04-31. Instead of returning an invalid result, the last
> valid day of the month, 2007-04-30, is selected instead.

Meaning this is working as intended. So without resorting to the vintage Date/Time api...

What is the correct way to get the number of months between two dates?

答案1

得分: 5

你可以使用YearMonth类来仅考虑年份和月份。 <sup>示例</sup>

System.out.println(
    "现在和稍后之间的月份差:"  + 
    ChronoUnit.MONTHS.between(
        YearMonth.from(now), 
        YearMonth.from(later)
    )
);

导入java.time.temporal.ChronoUnitjava.time.YearMonth

英文:

You can use the YearMonth class to only consider years and months. <sup>Demo</sup>

System.out.println(
    &quot;Months between now and later:&quot;  + 
    ChronoUnit.MONTHS.between(
        YearMonth.from(now), 
        YearMonth.from(later)
    )
);

Import java.time.temporal.ChronoUnit and java.time.YearMonth.

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  • 本文由 发表于 2020年8月5日 09:50:18
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