java学习：日期的运算

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
 
public class HelloWorld {
    
    public static void main(String[] args) {
    	
    	//指定时间输出格式
    	SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
       
        Date dt = new Date();
        System.out.println("当前时间：" + sdf.format(dt));
        
        Calendar rightNow = Calendar.getInstance();
        rightNow.setTime(dt);        
        
        rightNow.add(Calendar.YEAR,-1);//当时日期减1年
        System.out.println("减1年：" + sdf.format(rightNow.getTime()));
        
        rightNow.add(Calendar.MONTH,3);//(在刚才的结果上)再加3个月
        System.out.println("再加3个月：" + sdf.format(rightNow.getTime()));
        
        rightNow.add(Calendar.DAY_OF_YEAR,10);//(在刚才的结果上)再加10天        
        System.out.println("再加10天：" + sdf.format(rightNow.getTime()));
        

    }
}

 1         Date dt = new Date();        
 2         Calendar calendar = Calendar.getInstance();
 3         calendar.setTime(dt);     
 4            System.out.println("当前日期:" + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(dt));//当前日期:2013-02-28 16:22:37
 5         System.out.println("年:" + calendar.get(Calendar.YEAR));//年:2013
 6         System.out.println("月:" + (calendar.get(Calendar.MONTH)+1));//月:2 -注:月份是从0开始的
 7         System.out.println("日:" + calendar.get(Calendar.DATE));//日:28
 8         System.out.println("时(12小时制):" + calendar.get(Calendar.HOUR));//时:4(12小时制)
 9         System.out.println("时(24小时制):" + calendar.get(Calendar.HOUR_OF_DAY));//时:16(24小时制)
10         System.out.println("分:" + calendar.get(Calendar.MINUTE));//分:30
11         System.out.println("秒:" + calendar.get(Calendar.SECOND));//秒:19
12         System.out.println("本月第" + calendar.get(Calendar.WEEK_OF_MONTH) + "周");//本月第5周
13         System.out.println("本年第" + calendar.get(Calendar.WEEK_OF_YEAR) + "周");//本年第9周