JAVA时间工具类用法

1.获得N天前的TIMESTAMP

 Calendar cl = Calendar.getInstance();
 cl.add(Calendar.DAY_OF_YEAR, -7);
 Date date = cl.getTime();
 long time = date.getTime();
 Timestamp timestamp = new Timestamp(time);

2.程序启动当前时间和任务执行既定时间

 /*
 delegate time
 */
 String time="20:00";

 /*
 simpledateformat can parse String to Date, format Date to String
 */
 DateFormat dateFormat = new SimpleDateFormat("yy-MM-dd HH:mm");
 DateFormat dayFormat = new SimpleDateFormat("yy-MM-dd");

 Date curDate = dateFormat.parse(dayFormat.format(new Date()) + " " + time);

 Long current =System.currentTimeMillis();
 Long certain = curDate.getTime();