如何获取当前周和月的第一天？

Question

我有几个事件的日期，以 毫秒[1] 表示，我想知道当前周和当前月内的事件，但我不知道如何获取运行周的第一天（日/月/年）并将其转换为毫秒，与该月的第一天相同。

[1]Since January 1, 1970, 00:00:00 GMT

Answer 1

本周以毫秒为单位：

// get today and clear time of day
Calendar cal = Calendar.getInstance();
cal.set(Calendar.HOUR_OF_DAY, 0); // ! clear would not reset the hour of day !
cal.clear(Calendar.MINUTE);
cal.clear(Calendar.SECOND);
cal.clear(Calendar.MILLISECOND);

// get start of this week in milliseconds
cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());
System.out.println("Start of this week:       " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

// start of the next week
cal.add(Calendar.WEEK_OF_YEAR, 1);
System.out.println("Start of the next week:   " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

本月以毫秒为单位：

// get today and clear time of day
Calendar cal = Calendar.getInstance();
cal.set(Calendar.HOUR_OF_DAY, 0); // ! clear would not reset the hour of day !
cal.clear(Calendar.MINUTE);
cal.clear(Calendar.SECOND);
cal.clear(Calendar.MILLISECOND);

// get start of the month
cal.set(Calendar.DAY_OF_MONTH, 1);
System.out.println("Start of the month:       " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

// get start of the next month
cal.add(Calendar.MONTH, 1);
System.out.println("Start of the next month:  " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

Answer 2

可以在java.util.Calendar 的帮助下确定一周的第一天，如下所示：

Calendar calendar = Calendar.getInstance();
calendar.clear();
calendar.setTimeInMillis(timestamp);
while (calendar.get(Calendar.DAY_OF_WEEK) > calendar.getFirstDayOfWeek()) {
    calendar.add(Calendar.DATE, -1); // Substract 1 day until first day of week.
}
long firstDayOfWeekTimestamp = calendar.getTimeInMillis();

一个月的第一天可以确定如下：

Calendar calendar = Calendar.getInstance();
calendar.clear();
calendar.setTimeInMillis(timestamp);
while (calendar.get(Calendar.DATE) > 1) {
    calendar.add(Calendar.DATE, -1); // Substract 1 day until first day of month.
}
long firstDayOfMonthTimestamp = calendar.getTimeInMillis();

相当冗长，是的。

---

Java 7 将提供一个大大改进的日期和时间 API (JSR-310)。如果您还不能切换，那么您可以使用JodaTime，这样可以让一切变得不那么复杂：

DateTime dateTime = new DateTime(timestamp);
long firstDayOfWeekTimestamp = dateTime.withDayOfWeek(1).getMillis();

和

DateTime dateTime = new DateTime(timestamp);
long firstDayOfMonthTimestamp = dateTime.withDayOfMonth(1).getMillis();

Answer 3

java.time

Java 8 及更高版本中的 java.time 框架取代了旧的 java.util.Date/.Calendar 类。旧的课程已被证明是麻烦的、令人困惑的和有缺陷的。避开他们。

java.time 框架受到高度成功的 Joda-Time 库的启发，该库由 JSR 310 定义，由 ThreeTen-Extra 项目扩展，并在 Tutorial 中进行了解释。

Instant

Instant 类代表UTC 中时间轴上的一个时刻。

java.time 框架的分辨率为纳秒，或小数秒的 9 位数字。毫秒只是小数秒的 3 位数字。因为millisecond resolution 很常见，所以java.time 包含了一个方便的工厂方法。

long millisecondsSinceEpoch = 1446959825213L;
Instant instant = Instant.ofEpochMilli ( millisecondsSinceEpoch );

millisecondsSinceEpoch: 1446959825213 是即时的: 2015-11-08T05:17:05.213Z

ZonedDateTime

要考虑当前周和当前月，我们需要应用特定的时区。

ZoneId zoneId = ZoneId.of ( "America/Montreal" );
ZonedDateTime zdt = ZonedDateTime.ofInstant ( instant , zoneId );

在 zoneId: America/Montreal 即：2015-11-08T00:17:05.213-05:00[America/Montreal]

半开

在日期时间工作中，我们通常使用半开方法来定义时间跨度。开头是inclusive，而结尾是exclusive。我们没有尝试确定一周（或月）结束的最后一瞬间，而是获得下一周周（或月）的第一时刻。因此，一周从星期一的第一刻开始一直到但不包括下一个星期一的第一刻。

让我们在一周的第一天，也是最后一天。 java.time 框架包括一个工具，with 方法和ChronoField 枚举。

默认情况下，java.time 使用ISO 8601 标准。所以周一是一周的第一天 (1)，周日是最后一天 (7)。

ZonedDateTime firstOfWeek = zdt.with ( ChronoField.DAY_OF_WEEK , 1 ); // ISO 8601, Monday is first day of week.
ZonedDateTime firstOfNextWeek = firstOfWeek.plusWeeks ( 1 );

那一周从：2015-11-02T00:17:05.213-05:00[America/Montreal] 到 2015-11-09T00:17:05.213-05:00[America/Montreal]

哎呀！查看这些值的时间。我们想要一天的第一刻。由于夏令时 (DST) 或其他异常情况，一天中的第一时刻并不总是 00:00:00.000。所以我们应该让 java.time 代表我们进行调整。为此，我们必须通过 LocalDate 类。

ZonedDateTime firstOfWeek = zdt.with ( ChronoField.DAY_OF_WEEK , 1 ); // ISO 8601, Monday is first day of week.
firstOfWeek = firstOfWeek.toLocalDate ().atStartOfDay ( zoneId );
ZonedDateTime firstOfNextWeek = firstOfWeek.plusWeeks ( 1 );

该周从：2015-11-02T00:00-05:00[America/Montreal] 到 2015-11-09T00:00-05:00[America/Montreal]

这个月也一样。

ZonedDateTime firstOfMonth = zdt.with ( ChronoField.DAY_OF_MONTH , 1 );
firstOfMonth = firstOfMonth.toLocalDate ().atStartOfDay ( zoneId );
ZonedDateTime firstOfNextMonth = firstOfMonth.plusMonths ( 1 );

该月从：2015-11-01T00:00-04:00[America/Montreal] 到 2015-12-01T00:00-05:00[America/Montreal]

YearMonth

查看一对时刻是否在同一月份的另一种方法是检查相同的YearMonth 值。

例如，假设thisZdt 和thatZdt 都是ZonedDateTime 对象：

boolean inSameMonth = YearMonth.from( thisZdt ).equals( YearMonth.from( thatZdt ) ) ;

毫秒

我强烈建议不要以毫秒为单位进行日期时间工作。这确实是日期时间类倾向于在内部工作的方式，但我们有这些类是有原因的。处理从纪元开始的计数很笨拙，因为人类无法理解这些值，因此调试和记录很困难并且容易出错。而且，正如我们已经看到的，不同的解决方案可能正在发挥作用。旧的 Java 类和 Joda-Time 库使用毫秒，而 Postgres 等数据库使用微秒，现在 java.time 使用纳秒。

您会将文本作为位处理，还是让String、StringBuffer 和StringBuilder 等类处理此类细节？

但如果你坚持，从ZonedDateTime 得到一个Instant，然后从那个得到一个毫秒数。但请记住此调用可能意味着数据丢失。 ZonedDateTime/Instant 中可能包含的任何微秒或纳秒都将被截断（丢失）。

long millis = firstOfWeek.toInstant().toEpochMilli();  // Possible data loss.

---

关于java.time

java.time 框架内置于 Java 8 及更高版本中。这些类取代了麻烦的旧 legacy 日期时间类，例如 java.util.Date、Calendar 和 SimpleDateFormat。

要了解更多信息，请参阅Oracle Tutorial。并在 Stack Overflow 上搜索许多示例和解释。规格为JSR 310。

Joda-Time 项目现在位于maintenance mode，建议迁移到java.time 类。

您可以直接与您的数据库交换 java.time 对象。使用符合JDBC 4.2 或更高版本的JDBC driver。不需要字符串，不需要java.sql.* 类。

从哪里获得 java.time 类？

• Java SE 8、Java SE 9、Java SE 10、Java SE 11 和更高版本 - 具有捆绑实现的标准 Java API 的一部分。
  • Java 9 添加了一些小功能和修复。
• Java SE 6 和 Java SE 7
  • 大部分 java.time 功能在ThreeTen-Backport 中向后移植到 Java 6 和 7。
• Android
  • java.time 类的 Android 捆绑包实现的更高版本。
  • 对于早期的 Android (ThreeTenABP 项目适应 ThreeTen-Backport（如上所述）。见How to use ThreeTenABP…。

Answer 4

注意！

while (calendar.get(Calendar.DAY_OF_WEEK) > calendar.getFirstDayOfWeek()) {
    calendar.add(Calendar.DATE, -1); // Substract 1 day until first day of week.
}

是个好主意，但有一些问题： 例如，我来自乌克兰，而我所在国家/地区的 calendar.getFirstDayOfWeek() 是 2（星期一）。 今天是1（星期日）。在这种情况下，不调用 calendar.add。

因此，正确的方法是将“>”更改为“！=”：

while (calendar.get(Calendar.DAY_OF_WEEK) != calendar.getFirstDayOfWeek()) {...

Answer 5

您可以使用 java.time 包（从 Java8 和后期开始）获取一天/周/月的开始/结束。
下面的 util 类示例：

import org.junit.Test;

import java.time.DayOfWeek;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.ZoneId;
import java.time.format.DateTimeFormatter;
import java.time.temporal.TemporalAdjusters;
import java.util.Date;

public class DateUtil {
    private static final ZoneId DEFAULT_ZONE_ID = ZoneId.of("UTC");

    public static LocalDateTime startOfDay() {
        return LocalDateTime.now(DEFAULT_ZONE_ID).with(LocalTime.MIN);
    }

    public static LocalDateTime endOfDay() {
        return LocalDateTime.now(DEFAULT_ZONE_ID).with(LocalTime.MAX);
    }

    public static boolean belongsToCurrentDay(final LocalDateTime localDateTime) {
        return localDateTime.isAfter(startOfDay()) && localDateTime.isBefore(endOfDay());
    }

    //note that week starts with Monday
    public static LocalDateTime startOfWeek() {
        return LocalDateTime.now(DEFAULT_ZONE_ID)
                .with(LocalTime.MIN)
                .with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));
    }

    //note that week ends with Sunday
    public static LocalDateTime endOfWeek() {
        return LocalDateTime.now(DEFAULT_ZONE_ID)
                .with(LocalTime.MAX)
                .with(TemporalAdjusters.nextOrSame(DayOfWeek.SUNDAY));
    }

    public static boolean belongsToCurrentWeek(final LocalDateTime localDateTime) {
        return localDateTime.isAfter(startOfWeek()) && localDateTime.isBefore(endOfWeek());
    }

    public static LocalDateTime startOfMonth() {
        return LocalDateTime.now(DEFAULT_ZONE_ID)
                .with(LocalTime.MIN)
                .with(TemporalAdjusters.firstDayOfMonth());
    }

    public static LocalDateTime endOfMonth() {
        return LocalDateTime.now(DEFAULT_ZONE_ID)
                .with(LocalTime.MAX)
                .with(TemporalAdjusters.lastDayOfMonth());
    }

    public static boolean belongsToCurrentMonth(final LocalDateTime localDateTime) {
        return localDateTime.isAfter(startOfMonth()) && localDateTime.isBefore(endOfMonth());
    }

    public static long toMills(final LocalDateTime localDateTime) {
        return localDateTime.atZone(DEFAULT_ZONE_ID).toInstant().toEpochMilli();
    }

    public static Date toDate(final LocalDateTime localDateTime) {
        return Date.from(localDateTime.atZone(DEFAULT_ZONE_ID).toInstant());
    }

    public static String toString(final LocalDateTime localDateTime) {
        return localDateTime.format(DateTimeFormatter.ISO_DATE_TIME);
    }

    @Test
    public void test() {
        //day
        final LocalDateTime now = LocalDateTime.now();
        System.out.println("Now: " + toString(now) + ", in mills: " + toMills(now));
        System.out.println("Start of day: " + toString(startOfDay()));
        System.out.println("End of day: " + toString(endOfDay()));
        System.out.println("Does '" + toString(now) + "' belong to the current day? > " + belongsToCurrentDay(now));
        final LocalDateTime yesterday = now.minusDays(1);
        System.out.println("Does '" + toString(yesterday) + "' belong to the current day? > " + belongsToCurrentDay(yesterday));
        final LocalDateTime tomorrow = now.plusDays(1);
        System.out.println("Does '" + toString(tomorrow) + "' belong to the current day? > " + belongsToCurrentDay(tomorrow));
        //week
        System.out.println("Start of week: " + toString(startOfWeek()));
        System.out.println("End of week: " + toString(endOfWeek()));
        System.out.println("Does '" + toString(now) + "' belong to the current week? > " + belongsToCurrentWeek(now));
        final LocalDateTime previousWeek = now.minusWeeks(1);
        System.out.println("Does '" + toString(previousWeek) + "' belong to the current week? > " + belongsToCurrentWeek(previousWeek));
        final LocalDateTime nextWeek = now.plusWeeks(1);
        System.out.println("Does '" + toString(nextWeek) + "' belong to the current week? > " + belongsToCurrentWeek(nextWeek));
        //month
        System.out.println("Start of month: " + toString(startOfMonth()));
        System.out.println("End of month: " + toString(endOfMonth()));
        System.out.println("Does '" + toString(now) + "' belong to the current month? > " + belongsToCurrentMonth(now));
        final LocalDateTime previousMonth = now.minusMonths(1);
        System.out.println("Does '" + toString(previousMonth) + "' belong to the current month? > " + belongsToCurrentMonth(previousMonth));
        final LocalDateTime nextMonth = now.plusMonths(1);
        System.out.println("Does '" + toString(nextMonth) + "' belong to the current month? > " + belongsToCurrentMonth(nextMonth));
    }
}

测试输出：

Now: 2020-02-16T22:12:49.957, in mills: 1581891169957
Start of day: 2020-02-16T00:00:00
End of day: 2020-02-16T23:59:59.999999999
Does '2020-02-16T22:12:49.957' belong to the current day? > true
Does '2020-02-15T22:12:49.957' belong to the current day? > false
Does '2020-02-17T22:12:49.957' belong to the current day? > false
Start of week: 2020-02-10T00:00:00
End of week: 2020-02-16T23:59:59.999999999
Does '2020-02-16T22:12:49.957' belong to the current week? > true
Does '2020-02-09T22:12:49.957' belong to the current week? > false
Does '2020-02-23T22:12:49.957' belong to the current week? > false
Start of month: 2020-02-01T00:00:00
End of month: 2020-02-29T23:59:59.999999999
Does '2020-02-16T22:12:49.957' belong to the current month? > true
Does '2020-01-16T22:12:49.957' belong to the current month? > false
Does '2020-03-16T22:12:49.957' belong to the current month? > false

Answer 6

要获取该月的第一天，只需获取 Date 并将当前日期设置为该月的第一天。如果需要，清除小时、分钟、秒和毫秒。

private static Date firstDayOfMonth(Date date) {
   Calendar calendar = Calendar.getInstance();
   calendar.setTime(date);
   calendar.set(Calendar.DATE, 1);
   return calendar.getTime();
}

一周的第一天是一样的，但使用 Calendar.DAY_OF_WEEK 代替

private static Date firstDayOfWeek(Date date) {
   Calendar calendar = Calendar.getInstance();
   calendar.setTime(date);
   calendar.set(Calendar.DAY_OF_WEEK, 1);
   return calendar.getTime();
}

Answer 7

我为此创建了一些方法：

public static String catchLastDayOfCurrentWeek(String pattern) {
    Calendar cal = Calendar.getInstance();
    cal.set(Calendar.HOUR_OF_DAY, 0);
    cal.clear(Calendar.MINUTE);
    cal.clear(Calendar.SECOND);
    cal.clear(Calendar.MILLISECOND);

    cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());

    return calendarToString(cal, pattern);
}

public static String catchLastDayOfCurrentWeek(String pattern) {
    Calendar cal = Calendar.getInstance();
    cal.set(Calendar.HOUR_OF_DAY, 0);
    cal.clear(Calendar.MINUTE);
    cal.clear(Calendar.SECOND);
    cal.clear(Calendar.MILLISECOND);

    cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());

    cal.add(Calendar.WEEK_OF_YEAR, 1);
    cal.add(Calendar.MILLISECOND, -1);

    return calendarToString(cal, pattern);
}

public static String catchTheFirstDayOfThemonth(Integer month, pattern padrao) {
    Calendar cal = GregorianCalendar.getInstance();
    cal.setTime(new Date());
    cal.set(Calendar.MONTH, month);
    cal.set(Calendar.DAY_OF_MONTH, 1);

    return calendarToString(cal, pattern);
}

public static String catchTheLastDayOfThemonth(Integer month, String pattern) {
    Calendar cal = GregorianCalendar.getInstance();
    cal.setTime(new Date());
    cal.set(cal.get(Calendar.YEAR), month, cal.getActualMaximum(Calendar.DAY_OF_MONTH));

    return calendarToString(cal, pattern);
}

public static String calendarToString(Calendar calendar, String pattern) {
    if (calendar == null) {
        return "";
    }
    SimpleDateFormat format = new SimpleDateFormat(pattern, LocaleUtils.DEFAULT_LOCALE);
    return format.format(calendar.getTime());
}

你可以看到更多here。

Answer 8

在这种情况下：

// get today and clear time of day
Calendar cal = Calendar.getInstance();
cal.clear(Calendar.HOUR_OF_DAY);  <---- is the current hour not 0 hour
cal.clear(Calendar.MINUTE);
cal.clear(Calendar.SECOND);
cal.clear(Calendar.MILLISECOND);

因此 Calendar.HOUR_OF_DAY 返回 8、9、12、15、18 作为当前运行时间。 我认为最好通过以下方式更改此行：

c.set(Calendar.HOUR_OF_DAY,0);

这样一天总是从 0 点开始

Answer 9

获取下个月的第一个日期：-

SimpleDateFormat df = new SimpleDateFormat("MM-dd-yyyy");
String selectedDate="MM-dd-yyyy like 07-02-2018";
Date dt = df.parse(selectedDate);`enter code here`
calendar = Calendar.getInstance();
calendar.setTime(dt);
calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH) + 1);

String firstDate = df.format(calendar.getTime());
System.out.println("firstDateof next month ==>" + firstDate);

Answer 10

您应该能够将您的号码转换为 Java 日历，例如：

 Calendar.getInstance().setTimeInMillis(myDate);

从那里开始，比较应该不会太难。

Answer 11

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.Scanner;

/**
This Program will display day for, 1st and last days in a given month and year

@author Manoj Kumar Dunna
Mail Id : manojdunna@gmail.com
*/
public class DayOfWeek {
    public static void main(String[] args) {
        String strDate = null;
        int  year = 0, month = 0;
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter YYYY/MM: ");
        strDate = sc.next();
        Calendar cal = new GregorianCalendar();
        String [] date = strDate.split("/");
        year = Integer.parseInt(date[0]);
        month = Integer.parseInt(date[1]);
        cal.set(year, month-1, 1);
        System.out.println(new SimpleDateFormat("EEEE").format(cal.getTime()));
        cal.add(Calendar.MONTH, 1);
        cal.add(Calendar.DAY_OF_YEAR, -1);
        System.out.println(new SimpleDateFormat("EEEE").format(cal.getTime()));
    }
}

Answer 12

Simple Solution:
package com.util.calendarutil;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class CalUtil {
    public static void main(String args[]){
        DateFormat df = new SimpleDateFormat("dd/mm/yyyy");
        Date dt = null;
        try {
            dt = df.parse("23/01/2016");
        } catch (ParseException e) {
            System.out.println("Error");
        }

        Calendar cal = Calendar.getInstance();
        cal.setTime(dt);
        cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());
        Date startDate = cal.getTime();
        cal.add(Calendar.DATE, 6);
        Date endDate = cal.getTime();
        System.out.println("Start Date:"+startDate+"End Date:"+endDate);


    }

}

Answer 13

使用 Java 8 特性的单行解决方案

在 Java

中

LocalDateTime firstOfWeek = LocalDateTime.now().with(ChronoField.DAY_OF_WEEK, 1).toLocalDate().atStartOfDay(); // 2020-06-08 00:00 MONDAY
LocalDateTime firstOfMonth = LocalDateTime.now().with(ChronoField.DAY_OF_MONTH , 1).toLocalDate().atStartOfDay(); // 2020-06-01 00:00
// Convert to milliseconds: 
firstOfWeek.atZone(ZoneId.systemDefault()).toInstant().toEpochMilli();

在 Kotlin

中

val firstOfWeek = LocalDateTime.now().with(ChronoField.DAY_OF_WEEK, 1).toLocalDate().atStartOfDay() // 2020-06-08 00:00 MONDAY
val firstOfMonth = LocalDateTime.now().with(ChronoField.DAY_OF_MONTH , 1).toLocalDate().atStartOfDay() // 2020-06-01 00:00
// Convert to milliseconds: 
firstOfWeek.atZone(ZoneId.systemDefault()).toInstant().toEpochMilli()

Answer 14

我使用这个技巧来获取当月的第一天 注意顺序是 1 周日 2 星期一 3 周二 ....等等

Calendar cal = new GregorianCalendar();
int startDay = cal.get(Calendar.DAY_OF_YEAR) % 7 + 1;
System.out.println(startDay);

Answer 15

public static void main(String[] args) {
    System.out.println(getMonthlyEpochList(1498867199L,12,"Monthly"));

}

public static Map<String,String> getMonthlyEpochList(Long currentEpoch, int noOfTerms, String timeMode) {
    Map<String,String> map = new LinkedHashMap<String,String>();
    int month = 0;
    while(noOfTerms != 0) {
        Calendar calendar = Calendar.getInstance();         
        calendar.add(Calendar.MONTH, month);
        calendar.set(Calendar.DATE, calendar.getActualMinimum(Calendar.DAY_OF_MONTH));
        Date monthFirstDay = calendar.getTime();
        calendar.set(Calendar.DATE, calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
        Date monthLastDay = calendar.getTime();
        map.put(getMMYY(monthFirstDay.getTime()), monthFirstDay + ":" +monthLastDay);
        month--;
        noOfTerms--;
    }
    return map;     
}