java中日期的加减

public static String getFormatTenSecondTimeBefore(String time) {
        SimpleDateFormat format1 = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'.000Z'");
        SimpleDateFormat format2 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        String utcDate = "";
        try {
            Date date = format2.parse(time);//传入的time 格式是yyyy-mm-dd HH:mm:ss
            //下面两行是增加的代码。有疑问查询getUTC方法
            String formatTenSceondBefore = format2.format((new Date(date.getTime() - 10 * 1000)));
            date = format2.parse(formatTenSceondBefore);
            format1.setTimeZone(TimeZone.getTimeZone("utc"));
            utcDate = format1.format(date);
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return utcDate;

    }

//具体实现：java.text.SimpleDateFormat
//java.util.Date
       
    Date date=new Date();   
   SimpleDateFormat df=new SimpleDateFormat("yyyy-MM-dd");   
   System.out.println("三天前的日期：" + df.format(new Date(date.getTime() - 3* 24 * 60 * 60 * 1000)));  
   System.out.println("三天后的日期：" + df.format(new Date(date.getTime() + 3 * 24 * 60 * 60 * 1000)));