使用java.util.Calendar添加天数会产生奇怪的结果 [英] Adding days with java.util.Calendar gives strange results

问题描述

使用java.util.Calendar将一天添加到Date，并使用SimpleDateFormat显示结果，有时似乎会丢失一天（通常在三月），有时会跳过一天（十一月）。

Using java.util.Calendar to add a single day to a Date, and SimpleDateFormat to display the result, sometimes seems to lose a day (generally in March) and sometimes skips a day (in November).

下面的程序带有输出，说明了这个问题。请注意，我只是一次添加一天，然后跳过几个月并再添加几天。您将看到2008-03-09被打印两次，但跳过了2008-11-02。同样的事情发生在其他年份，但在不同的日子。我必须尝试找出导致问题的日子。

The program below, with output, illustrates the issue. Notice that I'm just adding one day at a time, then skipping a few months and adding a few more days. You'll see that 2008-03-09 gets printed twice, but 2008-11-02 is skipped. The same thing happens in other years, but on different days. I had to experiment to find the days that cause the problem.

如果我没有在SimpleDateFormat中将时区设置为UTC，则问题不会发生。我在美国中部时区的一台机器上运行它。

If I don't set the timezone to UTC in the SimpleDateFormat then the problem does not occur. I ran this on a machine in the US Central Time Zone.

这当然看起来像是Calendar或SimpleDateFormat中的一个错误，但我无法在任何地方找到它。有人解释这里发生了什么吗？

This certainly looks like a bug in Calendar or SimpleDateFormat, but I have not been able to find it documented anywhere. Anybody have an explanation of what is happening here?

该计划：

package mab;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;

public class CalendarHiccup2 {

    public static void main(String[] args) {
        addDays("2008-03-08");
        addDays("2009-03-07");
        addDays("2010-03-13");
    }

    public static void addDays(String dateString) {
        System.out.println("Got dateString: " + dateString);

        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        sdf.setTimeZone(TimeZone.getTimeZone("UTC"));

        Calendar calendar = Calendar.getInstance();
        try {
            calendar.setTime(sdf.parse(dateString));
            Date day1 = calendar.getTime();
            System.out.println("  day1 = " + sdf.format(day1));

            calendar.add(java.util.Calendar.DAY_OF_MONTH, 1);
            Date day2 = calendar.getTime();
            System.out.println("  day2 = " + sdf.format(day2));

            calendar.add(java.util.Calendar.DAY_OF_MONTH, 1);
            Date day3 = calendar.getTime();
            System.out.println("  day3 = " + sdf.format(day3));

            calendar.add(java.util.Calendar.DAY_OF_MONTH, 1);
            Date day4 = calendar.getTime();
            System.out.println("  day4 = " + sdf.format(day4));

            // Skipping a few days ahead:
            calendar.add(java.util.Calendar.DAY_OF_MONTH, 235);
            Date day5 = calendar.getTime();
            System.out.println("  day5 = " + sdf.format(day5));

            calendar.add(java.util.Calendar.DAY_OF_MONTH, 1);
            Date day6 = calendar.getTime();
            System.out.println("  day6 = " + sdf.format(day6));

            calendar.add(java.util.Calendar.DAY_OF_MONTH, 1);
            Date day7 = calendar.getTime();
            System.out.println("  day7 = " + sdf.format(day7));

            calendar.add(java.util.Calendar.DAY_OF_MONTH, 1);
            Date day8 = calendar.getTime();
            System.out.println("  day8 = " + sdf.format(day8));

        } catch (Exception e) {
        }
    }

}

输出：

Got dateString: 2008-03-08
  day1 = 2008-03-08
  day2 = 2008-03-09
  day3 = 2008-03-09
  day4 = 2008-03-10
  day5 = 2008-10-31
  day6 = 2008-11-01
  day7 = 2008-11-03
  day8 = 2008-11-04
Got dateString: 2009-03-07
  day1 = 2009-03-07
  day2 = 2009-03-08
  day3 = 2009-03-08
  day4 = 2009-03-09
  day5 = 2009-10-30
  day6 = 2009-10-31
  day7 = 2009-11-02
  day8 = 2009-11-03
Got dateString: 2010-03-13
  day1 = 2010-03-13
  day2 = 2010-03-14
  day3 = 2010-03-14
  day4 = 2010-03-15
  day5 = 2010-11-05
  day6 = 2010-11-06
  day7 = 2010-11-08
  day8 = 2010-11-09

推荐答案

这是由夏令时并且完全正确。

时间（北半球）通常在3月提前一小时，并在11月返回。

The time (on north hemisphere) is advanced one hour typically in March and moved back in November.

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