设置Java Calendar的值不会给出预期的日期时间 [英] Setting values of Java Calendar does not give expected date-time

问题描述

我有一个小时，分钟，日期和毫秒时间戳，我正在尝试创建一个表示时间的Date对象。时间戳在东部夏令时提供。

I have an hour, minute, date and millisecond timestamp, and am trying to create a Date object representing the time. The timestamp is provided in Eastern Daylight Time.

在解剖问题时，我创建了一些简单的测试代码，以查看发生的情况并观察了以下情况：

In dissecting the problem, I created some simple test code to see what was happening and have observed the following:

    Date today = new Date();
    int hour = 4, min  = 0, sec  = 0, ms   = 64;
    boolean print = true;

    Calendar cal = GregorianCalendar.getInstance();
    if(print)
        System.out.println("After initializing, time is: "+cal.getTime());
    cal.clear();
    if(print)
        System.out.println("After clearing, time is: "+cal.getTime());
    cal.setTime(today);
    if(print)
        System.out.println("After setting date, time is: "+cal.getTime());
    cal.set(Calendar.HOUR_OF_DAY,hour);
    if(print)
        System.out.println("After setting hour, time is: "+cal.getTime());
    cal.set(Calendar.MINUTE,min);
    if(print)
        System.out.println("After setting minute, time is: "+cal.getTime());
    cal.set(Calendar.SECOND,sec);
    if(print)
        System.out.println("After setting second, time is: "+cal.getTime());
    cal.set(Calendar.MILLISECOND,ms);
    if(print)
        System.out.println("After setting milliseconds, time is: "+cal.getTime());
    cal.setTimeZone(TimeZone.getTimeZone("EDT"));

    System.out.println("After setting time zone, time is: "+cal.getTime());

这会产生输出：

After initializing, time is: Tue Jan 07 16:01:59 EST 2014
After clearing, time is: Thu Jan 01 00:00:00 EST 1970
After setting date, time is: Tue Jan 07 16:01:59 EST 2014
After setting hour, time is: Tue Jan 07 04:01:59 EST 2014
After setting minute, time is: Tue Jan 07 04:00:59 EST 2014
After setting second, time is: Tue Jan 07 04:00:00 EST 2014
After setting milliseconds, time is: Tue Jan 07 04:00:00 EST 2014
After setting time zone, time is: Tue Jan 07 04:00:00 EST 2014

但是，如果我稍微更改代码：

However, if I change the code slightly:

boolean print = false;

我得到以下（不同）结果（！）

I get the following (different) result (!)

After setting time zone, time is: Mon Jan 06 23:00:00 EST 2014

有谁知道为什么会这样？

Does anyone know why this is happening?

推荐答案

你需要设定时间区域第一。请参阅下面GregorianCalendar.setTimeZone的定义：

You need to set the time zone first. See the definition of GregorianCalendar.setTimeZone below:

public void setTimeZone(TimeZone value)
{
    zone = value;
    sharedZone = false;
    /* Recompute the fields from the time using the new zone.  This also
     * works if isTimeSet is false (after a call to set()).  In that case
     * the time will be computed from the fields using the new zone, then
     * the fields will get recomputed from that.  Consider the sequence of
     * calls: cal.setTimeZone(EST); cal.set(HOUR, 1); cal.setTimeZone(PST).
     * Is cal set to 1 o'clock EST or 1 o'clock PST?  Answer: PST.  More
     * generally, a call to setTimeZone() affects calls to set() BEFORE AND
     * AFTER it up to the next call to complete().
     */
    areAllFieldsSet = areFieldsSet = false;
}

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