查找今天日期的前一天和今天日期之后的日期答案

【问题标题】：Find the day before today's date and after today's date查找今天日期的前一天和今天日期之后的日期
【发布时间】：2020-09-09 09:35:59
【问题描述】：

我创建了一个财务应用程序，我希望该应用程序向用户显示财务月份。财务月份通常代表用户获得工资或最大收入的日子。那一天可以是每月的第一天，每月的最后一天，或者只是每个月的 15 号。该值可在设置中配置。

我尝试运行在我的 ViewModel 中调用的两个方法，getFirstDayOfMonth 和 getLastDayOfMonth

为了更好地理解上下文，这里有一些例子，我们将以今天的日期作为参考。 2020 年 9 月 9 日。输入值是我从设置中读取的值，用户可以从中选择，是从 1 到 31 的数字。

示例：输入：5 输出：2020 年 9 月 5 日 00:00 开始，2020 年 10 月 4 日 23:59:59 结束

=================

输入：31 输出：2020 年 8 月 31 日 00:00 开始，2020 年 9 月 30 日 23:59:59

要注意的是，如果月份没有那一天，它会得到最左边的，例如如果选择 31 作为第一天并且该月份有 30 天，那么 30 将被计算为第一天，还有结束日期，如果选择了 31 并且我们在 2 月，并且它只有 28 天，则将选择 28。

到目前为止，我有这段代码，但我觉得它可以改进，也没有按预期工作。

fun getFirstDayOfMonth(date: LocalDateTime): Long {
    var tempDate = date
    val firstDayOfMonth = lastDay?.filter { it.isDigit() }!!.toInt()

    if (firstDayOfMonth < tempDate.dayOfMonth) {
        tempDate = tempDate.withDayOfMonth(firstDayOfMonth)
    } else if (firstDayOfMonth > tempDate.dayOfMonth) {
        tempDate = tempDate.minusMonths(1)
        if (tempDate.monthValue == 12) {
//I don't know why minusMonths does not work in the same way as plusMonths, when I write .plusMonths(1) it also change the year if I am in december, with minusMonths if I am in January it does not change the year to minus one year.
            tempDate = tempDate.minusYears(1)
        }

        if (firstDayOfMonth > tempDate.with(TemporalAdjusters.lastDayOfMonth()).dayOfMonth) {
            tempDate.withDayOfMonth(tempDate.with(TemporalAdjusters.lastDayOfMonth()).dayOfMonth)
        } else {
            tempDate = tempDate.withDayOfMonth(firstDayOfMonth)
        }
    }

    return tempDate.atZone(ZoneId.systemDefault()).toInstant().toEpochMilli()
}

fun getLastDayOfMonth(date: LocalDateTime): Long {
    var tempDate = date
    val firstDayOfMonth = lastDay?.filter { it.isDigit() }!!.toInt()

    if (firstDayOfMonth > tempDate.dayOfMonth && firstDayOfMonth <= tempDate.with(
            TemporalAdjusters.lastDayOfMonth()
        ).dayOfMonth
    ) {
        tempDate = tempDate.withDayOfMonth(firstDayOfMonth).minusDays(1)
    } else {
        tempDate = tempDate.plusMonths(1)
        if (firstDayOfMonth > tempDate.with(TemporalAdjusters.lastDayOfMonth()).dayOfMonth) {
            tempDate.withDayOfMonth(tempDate.with(TemporalAdjusters.lastDayOfMonth()).dayOfMonth)
        } else {
            tempDate = tempDate.withDayOfMonth(firstDayOfMonth).minusDays(1)
        }
    }

    return tempDate.atZone(ZoneId.systemDefault()).toInstant().toEpochMilli()
}

例如，我编写了一个运行今天日期的测试，只将月份从 1 月更改为 12 月。这是当前算法的输出。每月 1 日是用户选择的日期，该日期在应用中也是默认设置。

Todays date 09.01.2020
Running for month January
Running for 1 st of the month
01.01.2020
31.01.2020
===========================================
Todays date 09.02.2020
Running for month February
Running for 1 st of the month
01.02.2020
29.02.2020
===========================================
Todays date 09.03.2020
Running for month March
Running for 1 st of the month
01.03.2020
31.03.2020
===========================================
Todays date 09.04.2020
Running for month April
Running for 1 st of the month
01.04.2020
30.04.2020
===========================================
Todays date 09.05.2020
Running for month May
Running for 1 st of the month
01.05.2020
31.05.2020
===========================================
Todays date 09.06.2020
Running for month June
Running for 1 st of the month
01.06.2020
30.06.2020
===========================================
Todays date 09.07.2020
Running for month July
Running for 1 st of the month
01.07.2020
31.07.2020
===========================================
Todays date 09.08.2020
Running for month August
Running for 1 st of the month
01.08.2020
31.08.2020
===========================================
Todays date 09.09.2020
Running for month September
Running for 1 st of the month
01.09.2020
30.09.2020
===========================================
Todays date 09.10.2020
Running for month October
Running for 1 st of the month
01.10.2020
31.10.2020
===========================================
Todays date 09.11.2020
Running for month November
Running for 1 st of the month
01.11.2020
30.11.2020
===========================================
Todays date 09.12.2020
Running for month December
Running for 1 st of the month
01.12.2020
31.12.2021
===========================================

Process finished with exit code 0

也适用于 31 个选择

Todays date 09.01.2020
Running for month January
Running for 31 th of the month
31.12.2019
30.01.2020
===========================================
Todays date 09.02.2020
Running for month February
Running for 31 th of the month
31.01.2020
30.03.2020
===========================================
Todays date 09.03.2020
Running for month March
Running for 31 th of the month
09.02.2020
30.03.2020
===========================================
Todays date 09.04.2020
Running for month April
Running for 31 th of the month
31.03.2020
30.05.2020
===========================================
Todays date 09.05.2020
Running for month May
Running for 31 th of the month
09.04.2020
30.05.2020
===========================================
Todays date 09.06.2020
Running for month June
Running for 31 th of the month
31.05.2020
30.07.2020
===========================================
Todays date 09.07.2020
Running for month July
Running for 31 th of the month
09.06.2020
30.07.2020
===========================================
Todays date 09.08.2020
Running for month August
Running for 31 th of the month
31.07.2020
30.08.2020
===========================================
Todays date 09.09.2020
Running for month September
Running for 31 th of the month
31.08.2020
30.10.2020
===========================================
Todays date 09.10.2020
Running for month October
Running for 31 th of the month
09.09.2020
30.10.2020
===========================================
Todays date 09.11.2020
Running for month November
Running for 31 th of the month
31.10.2020
30.12.2021
===========================================
Todays date 09.12.2020
Running for month December
Running for 31 th of the month
09.11.2020
30.12.2021
===========================================

Process finished with exit code 0

【问题讨论】：

你真的需要时间吗？
不要将结束时间表示为最后一天的23:59:59。会有第二个不属于任何一个财政月，这是不愉快的，并且有可能导致错误。而是将财政月表示为从财政月开始（含）到下一个财政月开始不包括的半开区间。这是时间间隔的标准和推荐处理。
@OleV.V.好吧......用户在应用程序中添加这些值并使用当前时间。我认为在 23:59:59 向数据库添加新条目是完全不可能的
@MarianPavel - solution 是否有助于解决问题？

标签： date java-time localdate java.time.instant localdatetime

【解决方案1】：

您的计算中有一些错误，一旦您了解了下面给出的解决方案，您就可以很容易地发现这些错误。我在代码里放了足够多的cmets，可以帮助你快速理解。

import java.time.Instant;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.Month;
import java.time.YearMonth;
import java.time.ZoneOffset;
import java.time.temporal.ChronoUnit;
import java.time.temporal.TemporalAdjusters;

public class Main {
    public static void main(String[] args) {
        // Test for 31
        int startDay = 31;

        System.out.println(Instant.ofEpochMilli(getFirstDayOfMonth(startDay, YearMonth.of(2020, Month.SEPTEMBER))));
        System.out.println(Instant.ofEpochMilli(getLastDayOfMonth(startDay, YearMonth.of(2020, Month.SEPTEMBER))));
        
        System.out.println(Instant.ofEpochMilli(getFirstDayOfMonth(startDay, YearMonth.of(2020, Month.FEBRUARY))));
        System.out.println(Instant.ofEpochMilli(getLastDayOfMonth(startDay, YearMonth.of(2020, Month.FEBRUARY))));
        
        // Test for 30
        startDay = 30;
        System.out.println();
        System.out.println(Instant.ofEpochMilli(getFirstDayOfMonth(startDay, YearMonth.of(2020, Month.SEPTEMBER))));
        System.out.println(Instant.ofEpochMilli(getLastDayOfMonth(startDay, YearMonth.of(2020, Month.SEPTEMBER))));
        
        System.out.println(Instant.ofEpochMilli(getFirstDayOfMonth(startDay, YearMonth.of(2020, Month.FEBRUARY))));
        System.out.println(Instant.ofEpochMilli(getLastDayOfMonth(startDay, YearMonth.of(2020, Month.FEBRUARY))));
                
        // Test for 28
        startDay = 28;
        System.out.println();
        System.out.println(Instant.ofEpochMilli(getFirstDayOfMonth(startDay, YearMonth.of(2020, Month.SEPTEMBER))));
        System.out.println(Instant.ofEpochMilli(getLastDayOfMonth(startDay, YearMonth.of(2020, Month.SEPTEMBER))));
        
        System.out.println(Instant.ofEpochMilli(getFirstDayOfMonth(startDay, YearMonth.of(2020, Month.FEBRUARY))));
        System.out.println(Instant.ofEpochMilli(getLastDayOfMonth(startDay, YearMonth.of(2020, Month.FEBRUARY))));
    }

    static long getFirstDayOfMonth(int startDay, YearMonth ym) {
        // Get last day of the month
        int lastDayOfTheMonth = ym.getMonth().length(ym.isLeapYear());

        // Start of the day and on the first day of the month
        LocalDateTime ldt = LocalDate.of(ym.getYear(), ym.getMonth(), 1)
                            .atStartOfDay();

        if (startDay > lastDayOfTheMonth) {
            ldt = ldt.minusMonths(1) // Go back to the last month
                    .with(TemporalAdjusters.lastDayOfMonth()); // Adjust to the last day of the obtained month
        }
        return ldt.toInstant(ZoneOffset.UTC).toEpochMilli();
    }

    static long getLastDayOfMonth(int startDay, YearMonth ym) {
        return Instant.ofEpochMilli(getFirstDayOfMonth(startDay, ym))// Get the point to start with
                .plus(ym.getMonth().length(ym.isLeapYear()), ChronoUnit.DAYS)// Add the no. of days of the given month
                .atOffset(ZoneOffset.UTC)// Get OffsetDateTime in order to get LocalDate
                .toLocalDate()// Convert to LocalDate
                .atTime(LocalTime.of(23, 59, 59))// At 23:59:59
                .toInstant(ZoneOffset.UTC)// Convert to Instant
                .toEpochMilli();
    }
}

输出：

2020-08-31T00:00:00Z
2020-09-30T23:59:59Z
2020-01-31T00:00:00Z
2020-02-29T23:59:59Z

2020-09-01T00:00:00Z
2020-10-01T23:59:59Z
2020-01-31T00:00:00Z
2020-02-29T23:59:59Z

2020-09-01T00:00:00Z
2020-10-01T23:59:59Z
2020-02-01T00:00:00Z
2020-03-01T23:59:59Z

【讨论】：

【解决方案2】：

似乎这个 LocalDateTime 类具有您需要内置的所有花里胡哨的功能。检查 minusDays 和 plusDays 方法。

【讨论】：