Java 8计算两个日期之间的月份

Posted 2021-08-14 JavaEdge.

开始日期：“2021-08-31”
结束日期：“2021-11-30”

在上述两个日期之间的91天持续时间，期望代码返回3个月的持续时间，但是以下方法仅返回2个月。这是Java 8中的bug 吗？日期为91天，却仅返回2个月。

Period diff = Period.between(LocalDate.parse("2021-08-31"),
    LocalDate.parse("2021-11-30"));

long daysBetween = ChronoUnit.MONTHS.between(LocalDate.parse("2021-08-31"),
    LocalDate.parse("2021-11-30"));

我尝试使用Joda库而不是Java 8 API，它可以工作。它会返回3，看起来Java持续时间的月份计算也使用了天数。

LocalDate dateBefore= LocalDate.parse("2016-08-31");
    LocalDate dateAfter = LocalDate.parse("2016-11-30");
    int months = Months.monthsBetween(dateBefore, dateAfter).getMonths();
    System.out.println(months);

但我就想用 JDK 的 API。因此仍在寻找其他解决方案。

由于仅需要两个日期之间的月份数，请使用期间的文档来调整日期，如Jacob所述使用日期。只需将两个实例的日期设置为相同的值(每月的第一天)

Period diff = Period.between(
            LocalDate.parse("2021-08-31").withDayOfMonth(1),
            LocalDate.parse("2021-11-30").withDayOfMonth(1));
System.out.println(diff); //P3M

与其他解决方案相同：

long monthsBetween = ChronoUnit.MONTHS.between(
        LocalDate.parse("2021-08-31").withDayOfMonth(1),
        LocalDate.parse("2021-11-30").withDayOfMonth(1));
System.out.println(monthsBetween); //3

Period＃between的文档指出以下内容：

The start date is included, but the end date is not.

此外：

A month is considered if the end day-of-month is greater than or equal to the start day-of-month.

你的月底结束日期30不大于或等于您的月底开始日期31，因此不考虑第三个月。

注意参数名称：

public static Period between?(LocalDate startDateInclusive, LocalDate endDateExclusive)

要返回3个月，您可以将endDateExclusive增加一天。
因此，最好调整实施方式，使结束日期包括在内并获得所需的结果

Period diff = Period.between(LocalDate.parse("2016-08-31"),
                LocalDate.parse("2016-11-30").plusDays(1));
System.out.println("Months :" + diff.getMonths());
//Output -> Months : 3

向后兼容较旧的JDK 版本

public static int monthsBetween(Date d1, Date d2){
    if(d2==null || d1==null){
        return -1;//Error
    }
    Calendar m_calendar=Calendar.getInstance()
    m_calendar.setTime(d1);
    int nMonth1=12*m_calendar.get(Calendar.YEAR)+m_calendar.get(Calendar.MONTH);
    m_calendar.setTime(d2);
    int nMonth2=12*m_calendar.get(Calendar.YEAR)+m_calendar.get(Calendar.MONTH);
    return java.lang.Math.abs(nMonth2-nMonth1);
}

永远不要使用LocalDateTime来计算两个日期之间的月份，结果奇怪且不正确，请始终使用LocalDate！

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

public class TestMonthsDateTime {
    public static void main(String[] args) {
        /**------------------Date Time----------------------------*/
        LocalDateTime t1 = LocalDateTime.now();
        LocalDateTime t2 = LocalDateTime.now().minusMonths(3);
        long dateTimeDiff = ChronoUnit.MONTHS.between(t2, t1);  
        System.out.println("diff dateTime :" + dateTimeDiff); //diff dateTime : 2
        /**-------------------------Date----------------------------*/
        LocalDate t3 = LocalDate.now();
        LocalDate t4 = LocalDate.now().minusMonths(3);
        long dateDiff = ChronoUnit.MONTHS.between(t4, t3);
        System.out.println("diff date :"  +  dateDiff);//diff date : 3
    }
}

private static long durationMonths(LocalDate dateBefore, LocalDate dateAfter) {
        System.out.println(dateBefore+"  "+dateAfter);
        if (dateBefore.getDayOfMonth() > 28) {
            dateBefore = dateBefore.minusDays(5);
        } else if (dateAfter.getDayOfMonth() > 28) {
            dateAfter = dateAfter.minusDays(5);
        }
        return ChronoUnit.MONTHS.between(dateBefore, dateAfter);
    }