Java中的日历...我得到两个不同值的相同结果

Question

    Calendar cal = Calendar.getInstance();
    long now = cal.getTimeInMillis();
    int year = 2014;
    int month = 3;
    int date = 8;
    cal.set(year, month - 1, date);
    long dday = cal.getTimeInMillis();
    long count = (dday - now) / (1000 * 24 * 60 * 60);
    System.out.println((dday - now));
    System.out.println(count);

此代码计算出从现在到日期之间的天数。 但是，我在3月8日和3月9日得到了相同的结果。请帮忙！ 先感谢您。

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更详细的源代码示例，作者：Basil Bourque。

Calendar cal = Calendar.getInstance();
long nowInMillis = cal.getTimeInMillis();

cal.set( 2014, Calendar.MARCH, 8 );  // March 8, 2014   // year, month, day
long march_8_2014_inMillis = cal.getTimeInMillis();

cal.set( 2014,  Calendar.MARCH, 9 );  // March 9, 2014   // year, month, day
long march_9_2014_inMillis = cal.getTimeInMillis();

long daysTil8th = ( march_8_2014_inMillis - nowInMillis ) / ( 1000 * 24 * 60 * 60 );
long daysTil9th = ( march_9_2014_inMillis - nowInMillis ) / ( 1000 * 24 * 60 * 60 );

System.out.println( "( march_8_2014_inMillis - nowInMillis ) in artificial days: " + daysTil8th + ", in milliseconds: " + ( march_8_2014_inMillis - nowInMillis ) );
System.out.println( "( march_9_2014_inMillis - nowInMillis ) in artificial days: " + daysTil9th + ", in milliseconds: " + ( march_9_2014_inMillis - nowInMillis ) );

System.out.println( "( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ): " + ( ( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ) ) ) ;
System.out.println( "( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ) / ( 1000 * 60 * 60 ): " + ( ( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ) ) / ( 1000 * 60 * 60 ) ) ;

在西雅图，Java 7，Mac OS X 10.8.5上运行时2013-11-20T17：35：01.119-08：00（太平洋标准时间）......

( march_8_2014_inMillis - nowInMillis ) in artificial days: 108, in milliseconds: 9331200000
( march_9_2014_inMillis - nowInMillis ) in artificial days: 108, in milliseconds: 9414000000
( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ): 82800000
( march_9_2014_inMillis - nowInMillis ) - ( march_8_2014_inMillis - nowInMillis ) / ( 1000 * 60 * 60 ): 23

注意：对于我的时区， 夏令时开始于2014年3月9日星期日凌晨2:00。

问题：为什么上面的两行输出都是108 ？ 答案是DST，但究竟是怎么回事？

Answer 1

3月9日是DST将于2014年转换的日子。因此，计算结果是正确的，3月9日的差异是108天23小时，即108天。

Answer 2

正如Jim Garrison在他的正确答案中所说，基于整数的数学运算(1000 * 24 * 60 * 60)假设一天24小时。 对于我们这些在夏令时 （DST）的非理性练习的时区而言，情况并非如此。 在美国的时区8日至9日之间是23小时，而不是24小时。

得到教训：

• 永远不要做自己的日期和时间数学。
• 切勿使用Java中捆绑的java.util.Date和.Calendar。
• 总是使用像Joda-Time这样不错的库或Java 8中的新JSR 310 。

这是使用Joda-Time 2.3在Java 7中解决的相同问题，以及正确的输出。

// © 2013 Basil Bourque. This source code may be used freely forever by anyone taking full responsibility for doing so.

// Using US west coast time zone because March 9, 2014 at 02:00 is the start of DST Daylight Saving Time.
org.joda.time.DateTimeZone californiaTimeZone = org.joda.time.DateTimeZone.forID("America/Los_Angeles");

org.joda.time.DateTime now = new org.joda.time.DateTime(californiaTimeZone);

// Note the arbitrary choice of times (14 & 21 hours) assigned to new DateTimes.
org.joda.time.DateTime march_8_2014 = new org.joda.time.DateTime( 2014, 3, 8, 14, 0);
org.joda.time.DateTime march_9_2014 = new org.joda.time.DateTime( 2014, 3, 9, 21, 0);

int daysUntil8th = org.joda.time.Days.daysBetween( now , march_8_2014 ).getDays();
int daysUntil9th = org.joda.time.Days.daysBetween( now , march_9_2014 ).getDays();

// If you want whole days, call 'withTimeAtStartOfDate()' method.
// Don't get clever with trying to set a midnight time. Not every day in every time zone has a midnight.
int daysUntil8thStarts = org.joda.time.Days.daysBetween( now , march_8_2014.withTimeAtStartOfDay() ).getDays();
int daysUntil9thStarts = org.joda.time.Days.daysBetween( now , march_9_2014.withTimeAtStartOfDay() ).getDays();

System.out.println("Now in 'America/Los_Angeles' (Pacific Standard Time): " + now );

System.out.println("Days until " +  march_8_2014 + ": " + daysUntil8th );
System.out.println("Days until " + march_9_2014 + ": " + daysUntil9th );

System.out.println("Days until March 8, 2014 starts: " + daysUntil8thStarts );
System.out.println("Days until March 9, 2014 starts: " + daysUntil9thStarts );

跑的时候......

Now in 'America/Los_Angeles' (Pacific Standard Time): 2013-11-20T18:25:06.424-08:00
Days until 2014-03-08T14:00:00.000-08:00: 107
Days until 2014-03-09T21:00:00.000-07:00: 109
Days until March 8, 2014 starts: 107
Days until March 9, 2014 starts: 108

注意：时间会影响天数。 请注意107 109 107相对于108＆进行了两天的差与1天差。