在开始之前，如果要了解Java中的时区操作的基本概念和事例，可以参见另一篇博客：Java时区处理之Date,Calendar,TimeZone,SimpleDateFormat

一、夏令时概述：

Daylight Saving Time：DST
    夏令时，又称“日光节约时制”或“夏时制”，是一种为节约能源而人为规定地方时间的制度，在这一制度实行期间所采用的统一时间称为“夏令时间”。一般在天亮早的夏季人为将时间提前一小时，可以使人早起早睡，减少照明量，以充分利用光照资源，从而节约照明用电。各个采纳夏令时的国家具体规定不同。目前全世界有近110个国家每年要实行夏令时。（各时区多数位于其理想边界之西，导致实际上全年实施夏令时。）

1986年至1991年，中华人民共和国在全国范围实行了六年夏令时，每年从4月中旬的第一个星期日2时整（北京时间）到9月中旬第一个星期日的凌晨2时 整（北京夏令时）。除1986年因是实行夏令时的第一年，从5月4日开始到9月14日结束外，其它年份均按规定的时段施行。1992年4月5日后不再实 行。其实，1916年，德国首先实行夏令时，英国因为怕德国会从中得到更大的效益，因此紧跟着也采取了夏令时。日本现在没有实行夏令时，但是在美国驻军的1948年到1951年使用了夏令时。另外自2011年3月27日开始俄罗斯永久使用夏令时，把时间拨快一小时，不再调回。 夏令时会导致某一天多出一个小时，或者少出一个小时。

二、实例

1、先来看一个有趣的实例：1986年5月4号0点不见了

package com.wsheng.aggregator.timezone;import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;/*** * 1986年5月4号的0点不见了* * @author Josh Wang(Sheng)* * @email  swang6@ebay.com* */
public class DST1 {public static final DateFormat Y2MD_HMS = new SimpleDateFormat("yy-MM-dd HH:mm:ss");public static void main(String[] args) throws Exception {Date d = DST1.Y2MD_HMS.parse("1986-5-4 0:0:0");System.out.println(d); }}

 执行结果：

Sun May 04 01:00:00 CDT 1986

从上面的概述可以知道，这是因为中国在当天还在使用夏令时，时间被拨快了1个小时。

2、再来一个实例：预期的0点也没有了

package com.wsheng.aggregator.timezone;import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;/*** @author Josh Wang(Sheng)* * @email  swang6@ebay.com* */
public class DST2 {public static void main(String[] args) throws Exception {DateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:SS");  String sTime = "1991-04-07 00:00:00";  Date time = sdf.parse(sTime);  Calendar cd = Calendar.getInstance();  cd.setTime(time);  cd.add(Calendar.DATE, 7);  time = cd.getTime();  System.out.println(sdf.format(time)); }}

 预期结果：1991-04-14 00:00:00

 实际结果：  1991-04-14 01:00:00

从上面的分析也很容易得出，这也是由于当时的时间中国正在实习夏令时，使用 Calendar.DST_OFFSET 可以获得偏移量，其实夏令时是通过 TimeZone 的子类 sun.util.calendar.ZoneInfo 实现的。 ZoneInfo 中的数据存放于 %JRE_HOME%/lib/zi 目录中。（注意，是在jre的目录下）我们机器一般采用的时区名称为 Asia/Shanghai，相对应的时区信息文件位于：%JRE_HOME%/lib/zi/Asia/Shanghai 中，这是一个二进制文件。可通过 sun.util.calendar.ZoneInfoFile 的解析，可以获得需要采用夏令时的时间为：

  1940-06-03 01:00:00 ~ 1940-09-30 23:00:001941-03-16 01:00:00 ~ 1941-09-30 23:00:001986-05-04 01:00:00 ~ 1986-09-13 23:00:001987-04-12 01:00:00 ~ 1987-09-12 23:00:001988-04-10 01:00:00 ~ 1988-09-10 23:00:001989-04-16 01:00:00 ~ 1989-09-16 23:00:001990-04-15 01:00:00 ~ 1990-09-15 23:00:001991-04-14 01:00:00 ~ 1991-09-14 23:00:00

 前段表示那一天开始的时间，也就是说那一天的 0 点在该时区中是不存在。

后段表示那一天结束的时间，由于每个时区的夏令时都不一样，编程语言中的夏令时算法比较复杂，如在北京时区：

如果当前时间为：1986-09-13 23:00:00，增加 1 小时后为 1986-09-14 00:00:00
如果当前时间为：1986-09-13 22:00:00，增加 2 小时后为 1986-09-13 23:00:00，因为 1986-09-13 23:00:00 是夏令时结束时间，时钟需要回拨。

3、更具体的例子说明：美国洛杉矶的夏令时和冬令时

3.1、Java中不是每天都是标准的24个小时，可能是23，也可能是25。23小时和25小时就是夏令时、冬令时引起的。

3.2、下面用一个实例来判断中国和洛杉矶在1970年以来到从开始到往后的5年内对夏令时和冬令时的使用情况：

package com.wsheng.aggregator.timezone;import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;/*** 测试看看中国和美国洛杉矶在1970年以来到从现在开始的5年后的时间里，什么时候* 使用夏令时，什么时候没有使用夏令时* * @author Josh Wang(Sheng)* * @email  swang6@ebay.com* */
public class DST3 {private static void testDayTime(TimeZone timeZone) {System.out.println("Time Zone is : " + timeZone.getDisplayName() + " : " + timeZone.getID());  DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); Calendar start = Calendar.getInstance(timeZone);  start.setTime(new Date(0)); // UTC 1970-01-01System.out.println("start=" + df.format(start.getTime()));  // will print: start=1970-01-01 08:00:00Calendar end = Calendar.getInstance(timeZone);end.add(Calendar.YEAR, 5);System.out.println("end=" + df.format(end.getTime()));boolean find = false;for (long i = start.getTimeInMillis(); i < end.getTimeInMillis(); i = start.getTimeInMillis()) {start.add(Calendar.DATE, 1); // add one dayif ((start.getTimeInMillis() - i) % (24 * 3600 * 1000L) != 0) { // 是否能被24整除find = true;System.out.println("from " + df.format(new Date(i)) + " to " + df.format(start.getTime()) + " has "  + (start.getTimeInMillis() - i) + "ms" + "[" + (start.getTimeInMillis() - i) / (3600 * 1000L)  + "hours]");}}if (!find) {  System.out.println("Every day is ok.");  } }public static void main(String[] args) {TimeZone timeZone = TimeZone.getDefault();  DST3.testDayTime(timeZone);System.out.println(" -------------------- ");timeZone = TimeZone.getTimeZone("GMT");  DST3.testDayTime(timeZone);System.out.println(" -------------------- ");timeZone = TimeZone.getTimeZone("America/Los_Angeles");  DST3.testDayTime(timeZone);  }}

输出结果：

Time Zone is : China Standard Time : Asia/Shanghai
start=1970-01-01 08:00:00
end=2020-07-26 16:24:55
from 1986-05-03 08:00:00 to 1986-05-04 08:00:00 has 82800000ms[23hours]
from 1986-09-13 08:00:00 to 1986-09-14 08:00:00 has 90000000ms[25hours]
from 1987-04-11 08:00:00 to 1987-04-12 08:00:00 has 82800000ms[23hours]
from 1987-09-12 08:00:00 to 1987-09-13 08:00:00 has 90000000ms[25hours]
from 1988-04-09 08:00:00 to 1988-04-10 08:00:00 has 82800000ms[23hours]
from 1988-09-10 08:00:00 to 1988-09-11 08:00:00 has 90000000ms[25hours]
from 1989-04-15 08:00:00 to 1989-04-16 08:00:00 has 82800000ms[23hours]
from 1989-09-16 08:00:00 to 1989-09-17 08:00:00 has 90000000ms[25hours]
from 1990-04-14 08:00:00 to 1990-04-15 08:00:00 has 82800000ms[23hours]
from 1990-09-15 08:00:00 to 1990-09-16 08:00:00 has 90000000ms[25hours]
from 1991-04-13 08:00:00 to 1991-04-14 08:00:00 has 82800000ms[23hours]
from 1991-09-14 08:00:00 to 1991-09-15 08:00:00 has 90000000ms[25hours]--------------------
Time Zone is : Greenwich Mean Time : GMT
start=1970-01-01 08:00:00
end=2020-07-26 16:24:55
Every day is ok.--------------------
Time Zone is : Pacific Standard Time : America/Los_Angeles
start=1970-01-01 08:00:00
end=2020-07-26 16:24:55
from 1970-04-26 08:00:00 to 1970-04-27 07:00:00 has 82800000ms[23hours]
from 1970-10-25 07:00:00 to 1970-10-26 08:00:00 has 90000000ms[25hours]
from 1971-04-25 08:00:00 to 1971-04-26 07:00:00 has 82800000ms[23hours]
from 1971-10-31 07:00:00 to 1971-11-01 08:00:00 has 90000000ms[25hours]
from 1972-04-30 08:00:00 to 1972-05-01 07:00:00 has 82800000ms[23hours]
from 1972-10-29 07:00:00 to 1972-10-30 08:00:00 has 90000000ms[25hours]
from 1973-04-29 08:00:00 to 1973-04-30 07:00:00 has 82800000ms[23hours]
from 1973-10-28 07:00:00 to 1973-10-29 08:00:00 has 90000000ms[25hours]
from 1974-01-06 08:00:00 to 1974-01-07 07:00:00 has 82800000ms[23hours]
from 1974-10-27 07:00:00 to 1974-10-28 08:00:00 has 90000000ms[25hours]
from 1975-02-23 08:00:00 to 1975-02-24 07:00:00 has 82800000ms[23hours]
from 1975-10-26 07:00:00 to 1975-10-27 08:00:00 has 90000000ms[25hours]
from 1976-04-25 08:00:00 to 1976-04-26 07:00:00 has 82800000ms[23hours]
from 1976-10-31 07:00:00 to 1976-11-01 08:00:00 has 90000000ms[25hours]
from 1977-04-24 08:00:00 to 1977-04-25 07:00:00 has 82800000ms[23hours]
from 1977-10-30 07:00:00 to 1977-10-31 08:00:00 has 90000000ms[25hours]
from 1978-04-30 08:00:00 to 1978-05-01 07:00:00 has 82800000ms[23hours]
from 1978-10-29 07:00:00 to 1978-10-30 08:00:00 has 90000000ms[25hours]
from 1979-04-29 08:00:00 to 1979-04-30 07:00:00 has 82800000ms[23hours]
from 1979-10-28 07:00:00 to 1979-10-29 08:00:00 has 90000000ms[25hours]
from 1980-04-27 08:00:00 to 1980-04-28 07:00:00 has 82800000ms[23hours]
from 1980-10-26 07:00:00 to 1980-10-27 08:00:00 has 90000000ms[25hours]
from 1981-04-26 08:00:00 to 1981-04-27 07:00:00 has 82800000ms[23hours]
from 1981-10-25 07:00:00 to 1981-10-26 08:00:00 has 90000000ms[25hours]
from 1982-04-25 08:00:00 to 1982-04-26 07:00:00 has 82800000ms[23hours]
from 1982-10-31 07:00:00 to 1982-11-01 08:00:00 has 90000000ms[25hours]
from 1983-04-24 08:00:00 to 1983-04-25 07:00:00 has 82800000ms[23hours]
from 1983-10-30 07:00:00 to 1983-10-31 08:00:00 has 90000000ms[25hours]
from 1984-04-29 08:00:00 to 1984-04-30 07:00:00 has 82800000ms[23hours]
from 1984-10-28 07:00:00 to 1984-10-29 08:00:00 has 90000000ms[25hours]
from 1985-04-28 08:00:00 to 1985-04-29 07:00:00 has 82800000ms[23hours]
from 1985-10-27 07:00:00 to 1985-10-28 08:00:00 has 90000000ms[25hours]
from 1986-04-27 08:00:00 to 1986-04-28 07:00:00 has 82800000ms[23hours]
from 1986-10-26 07:00:00 to 1986-10-27 08:00:00 has 90000000ms[25hours]
from 1987-04-05 08:00:00 to 1987-04-06 07:00:00 has 82800000ms[23hours]
from 1987-10-25 07:00:00 to 1987-10-26 08:00:00 has 90000000ms[25hours]
from 1988-04-03 08:00:00 to 1988-04-04 07:00:00 has 82800000ms[23hours]
from 1988-10-30 07:00:00 to 1988-10-31 08:00:00 has 90000000ms[25hours]
from 1989-04-02 08:00:00 to 1989-04-03 07:00:00 has 82800000ms[23hours]
from 1989-10-29 07:00:00 to 1989-10-30 08:00:00 has 90000000ms[25hours]
from 1990-04-01 08:00:00 to 1990-04-02 07:00:00 has 82800000ms[23hours]
from 1990-10-28 07:00:00 to 1990-10-29 08:00:00 has 90000000ms[25hours]
from 1991-04-07 08:00:00 to 1991-04-08 07:00:00 has 82800000ms[23hours]
from 1991-10-27 07:00:00 to 1991-10-28 08:00:00 has 90000000ms[25hours]
from 1992-04-05 08:00:00 to 1992-04-06 07:00:00 has 82800000ms[23hours]
from 1992-10-25 07:00:00 to 1992-10-26 08:00:00 has 90000000ms[25hours]
from 1993-04-04 08:00:00 to 1993-04-05 07:00:00 has 82800000ms[23hours]
from 1993-10-31 07:00:00 to 1993-11-01 08:00:00 has 90000000ms[25hours]
from 1994-04-03 08:00:00 to 1994-04-04 07:00:00 has 82800000ms[23hours]
from 1994-10-30 07:00:00 to 1994-10-31 08:00:00 has 90000000ms[25hours]
from 1995-04-02 08:00:00 to 1995-04-03 07:00:00 has 82800000ms[23hours]
from 1995-10-29 07:00:00 to 1995-10-30 08:00:00 has 90000000ms[25hours]
from 1996-04-07 08:00:00 to 1996-04-08 07:00:00 has 82800000ms[23hours]
from 1996-10-27 07:00:00 to 1996-10-28 08:00:00 has 90000000ms[25hours]
from 1997-04-06 08:00:00 to 1997-04-07 07:00:00 has 82800000ms[23hours]
from 1997-10-26 07:00:00 to 1997-10-27 08:00:00 has 90000000ms[25hours]
from 1998-04-05 08:00:00 to 1998-04-06 07:00:00 has 82800000ms[23hours]
from 1998-10-25 07:00:00 to 1998-10-26 08:00:00 has 90000000ms[25hours]
from 1999-04-04 08:00:00 to 1999-04-05 07:00:00 has 82800000ms[23hours]
from 1999-10-31 07:00:00 to 1999-11-01 08:00:00 has 90000000ms[25hours]
from 2000-04-02 08:00:00 to 2000-04-03 07:00:00 has 82800000ms[23hours]
from 2000-10-29 07:00:00 to 2000-10-30 08:00:00 has 90000000ms[25hours]
from 2001-04-01 08:00:00 to 2001-04-02 07:00:00 has 82800000ms[23hours]
from 2001-10-28 07:00:00 to 2001-10-29 08:00:00 has 90000000ms[25hours]
from 2002-04-07 08:00:00 to 2002-04-08 07:00:00 has 82800000ms[23hours]
from 2002-10-27 07:00:00 to 2002-10-28 08:00:00 has 90000000ms[25hours]
from 2003-04-06 08:00:00 to 2003-04-07 07:00:00 has 82800000ms[23hours]
from 2003-10-26 07:00:00 to 2003-10-27 08:00:00 has 90000000ms[25hours]
from 2004-04-04 08:00:00 to 2004-04-05 07:00:00 has 82800000ms[23hours]
from 2004-10-31 07:00:00 to 2004-11-01 08:00:00 has 90000000ms[25hours]
from 2005-04-03 08:00:00 to 2005-04-04 07:00:00 has 82800000ms[23hours]
from 2005-10-30 07:00:00 to 2005-10-31 08:00:00 has 90000000ms[25hours]
from 2006-04-02 08:00:00 to 2006-04-03 07:00:00 has 82800000ms[23hours]
from 2006-10-29 07:00:00 to 2006-10-30 08:00:00 has 90000000ms[25hours]
from 2007-03-11 08:00:00 to 2007-03-12 07:00:00 has 82800000ms[23hours]
from 2007-11-04 07:00:00 to 2007-11-05 08:00:00 has 90000000ms[25hours]
from 2008-03-09 08:00:00 to 2008-03-10 07:00:00 has 82800000ms[23hours]
from 2008-11-02 07:00:00 to 2008-11-03 08:00:00 has 90000000ms[25hours]
from 2009-03-08 08:00:00 to 2009-03-09 07:00:00 has 82800000ms[23hours]
from 2009-11-01 07:00:00 to 2009-11-02 08:00:00 has 90000000ms[25hours]
from 2010-03-14 08:00:00 to 2010-03-15 07:00:00 has 82800000ms[23hours]
from 2010-11-07 07:00:00 to 2010-11-08 08:00:00 has 90000000ms[25hours]
from 2011-03-13 08:00:00 to 2011-03-14 07:00:00 has 82800000ms[23hours]
from 2011-11-06 07:00:00 to 2011-11-07 08:00:00 has 90000000ms[25hours]
from 2012-03-11 08:00:00 to 2012-03-12 07:00:00 has 82800000ms[23hours]
from 2012-11-04 07:00:00 to 2012-11-05 08:00:00 has 90000000ms[25hours]
from 2013-03-10 08:00:00 to 2013-03-11 07:00:00 has 82800000ms[23hours]
from 2013-11-03 07:00:00 to 2013-11-04 08:00:00 has 90000000ms[25hours]
from 2014-03-09 08:00:00 to 2014-03-10 07:00:00 has 82800000ms[23hours]
from 2014-11-02 07:00:00 to 2014-11-03 08:00:00 has 90000000ms[25hours]
from 2015-03-08 08:00:00 to 2015-03-09 07:00:00 has 82800000ms[23hours]
from 2015-11-01 07:00:00 to 2015-11-02 08:00:00 has 90000000ms[25hours]
from 2016-03-13 08:00:00 to 2016-03-14 07:00:00 has 82800000ms[23hours]
from 2016-11-06 07:00:00 to 2016-11-07 08:00:00 has 90000000ms[25hours]
from 2017-03-12 08:00:00 to 2017-03-13 07:00:00 has 82800000ms[23hours]
from 2017-11-05 07:00:00 to 2017-11-06 08:00:00 has 90000000ms[25hours]
from 2018-03-11 08:00:00 to 2018-03-12 07:00:00 has 82800000ms[23hours]
from 2018-11-04 07:00:00 to 2018-11-05 08:00:00 has 90000000ms[25hours]
from 2019-03-10 08:00:00 to 2019-03-11 07:00:00 has 82800000ms[23hours]
from 2019-11-03 07:00:00 to 2019-11-04 08:00:00 has 90000000ms[25hours]
from 2020-03-08 08:00:00 to 2020-03-09 07:00:00 has 82800000ms[23hours]

从输出结果可以看出：中国时区在1992年后就废除了夏令时、冬令时，GMT时区是不存在夏令时、冬令时，而美国洛杉矶一直在用夏令时、冬令时。

4、事例：好奇一天怎么会变23，25小时，继续试验。

在开始该实例时，有必要交代一下美国的时区，美国横跨西五区至西十区，共六个时区。每个时区对应一个标准时间，从东向西分别为东部时间(EST)(西五区时间)、中部时间(CST)(西六区时间)、山地时间(MST)(西七区时间)、太平洋时间(西部时间)(PST)(西八区时间)、阿拉斯加时间(AKST)(西九区时间)和夏威夷时间(HST)(西十区时间)，按照“东早西晚”的规律，各递减一小时。（由于美国使用夏令时，所以这些时区可以看做是冬令时，即夏令时结束后回拨回来的标准时间， 冬令时是在冬天使用的标准时间。在使用日光节约时制的地区，夏天时钟拨快一小时，冬天再拨回来。这时采用的是标准时间，也就是冬令时。）

如上标准时间（冬令时）一 一对应的夏令时为：

比如对于太平洋时区而言，标准时区（冬令时时区）为PST，夏令时时区为PDT。这个地方需要注意的是CST（中部时区），CST除了代表美国的西部时区外，还可以代表别的3个时区，即CST却同时可以代表如下 4 个不同的时区：

Central Standard Time (USA) UT-6:00
Central Standard Time (Australia) UT+9:30
China Standard Time UT+8:00
Cuba Standard Time UT-4:00  

可见，CST可以同时表示美国，澳大利亚，中国，古巴四个国家的标准时间。 有时做前后端开发的时候会碰到一些奇怪的问题，比如：

在服务器端通过 Java 获取当前时间为 Fri Aug 28 09:37:46 CST 2009， 转化为
GMT时间为：28 Aug 2009 01:37:46 GMT，也就是说GMT时间加上 8 个小时等于CST表示的时间， 那这个
CST不就是北京时间么，因为我们是在东八区的。 一切看起来很正常，不过在客户端用JavaScript解析这个时间就有问题了： 
    // Fri Aug 28 2009 23:37:46 GMT+0800
   new Date('Fri Aug 28 09:37:46 CST 2009').toString();
    好奇怪，这次GMT和CST表示的时间居然相差整整 14 个小时？

这就是因为CST可以代表多个时区引起来的，通过 Java 获取的CST时间用的是China Standard Time，而客户端JavaScript则默认采用的是美国的中部时间，所以整整相差了14个小时，
所以将 Fri Aug 28 09:37:46 CST 2009 加上 6 个小时，再加上 8 个小时，就等于 Fri Aug 28 2009 23:37:46
GMT+0800 
 可见，在以后的编程中为了避免错误，还是不要使用CST时间，而尽量采用GMT时间。

下面回到我们要说的例子上来：

package com.wsheng.aggregator.timezone;import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.TimeZone;/*** 为什么一天会变成23、25小时* * @author Josh Wang(Sheng)* * @email  swang6@ebay.com* */
public class DST4 {private static DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); public static void main(String[] args) {// 特殊时间点  long abc = 1414918799000l;  printDate(abc);  abc = 1414918800000l;  printDate(abc);  }private static void printDate(long now) {  // 中国时间  // TimeZone.setDefault(TimeZone.getTimeZone("Asia/Shanghai"));  // System.out.println(new Date(now).toString());  // System.out.println(new Date(now).toLocaleString());  // 美国时间  TimeZone.setDefault(TimeZone.getTimeZone("America/Los_Angeles"));  Date date = new Date(now);System.out.println(date);  System.out.println(df.format(date));  System.out.println("============================================================");  }  }

执行结果：

Sun Nov 02 01:59:59 PDT 2014
2014-11-02 16:59:59
============================================================
Sun Nov 02 01:00:00 PST 2014
2014-11-02 17:00:00
============================================================

发现long值为1414918799000l，美国时间是2014-11-2 1:59:59；但增加一秒1414918800000l，美国时间却是2014-11-2 1:00:00，这不是错误，仔细观察下你会发现，1414918799000l是夏令时PDT的 Sun Nov 02 01:59:59 PDT 2014；而1414918800000l是冬令时PST的 Sun Nov 02 01:00:00 PST 2014。

至此问题问题就明白了！！！

解决方案：

在Java中，想到Date和时区有关系，那只要在日期转换前设置下时区就解决问题了。

    private static String conventTime(long time) {  // 中国时间  // TimeZone.setDefault(TimeZone.getTimeZone("Asia/Shanghai"));  // 美国时间  TimeZone.setDefault(TimeZone.getTimeZone("America/Los_Angeles"));  return new Date(time).toLocaleString();  }