题目9： 2008年5月31日， 往前倒30天是哪天？ （视频下载） （全部书籍）
本章源码
import java.util.*;
public class Test {
    public static void main(String[] args) {
        Date date;
        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, 2008);
        cal.set(Calendar.MONTH, 4);
        cal.set(Calendar.DAY_OF_MONTH, 31);
        date = cal.getTime();
/* getTime()返回，从January 1, 1970, 00:00:00这个时候，到现在的毫秒数，
public long getTime() Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this Date object.
，60l表明是长整型， 如果用60的话就是int类型，马克-to-win 会溢出的。 这样we must use 60l*60l*24l*30l*1000l, while we can not use 60*60*24*30*1000, otherwise it will overflow（溢出）.
*/
        long myTime = date.getTime() - 60l * 60l * 24l * 30l * 1000l;
/* public void setTime(long time)Sets this Date object to represent a
point in time that is time milliseconds after January 1, 1970
00:00:00 GMT. */

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详情请见：http://www.mark-to-win.com/index.html?content=JavaBeginner/javaUrl.html&chapter=JavaBeginner/JavaBeginner2_web.html#DateBackwards