Java中日期时间的处理

1. 获取系统时间可以通过Calendar或者Date默认构造函数或者实例得到，不过现在日期主要还是借助于Calendar来处理。另外还可以得到当前日期是一周的第几天（从星期天开始），一月以及一年的第几天

	public static String getCurrentTime(){
		Calendar c = Calendar.getInstance();
		//HH 24 hour & hh 12 hour, E - week
		SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy E HH:mm:ss");
		return sdf.format(c.getTime());
	}
	public static void printTodayInfo(){
		Date date = new Date();
		Calendar c = Calendar.getInstance();
		c.setTime(date);
		System.out.println("Time for today:"+date.toString());
		System.out.println("Day for today :"+c.get(Calendar.DAY_OF_WEEK));
		System.out.println("Time for today:"+c.get(Calendar.DAY_OF_MONTH));
		System.out.println("Time for today:"+c.get(Calendar.DAY_OF_YEAR));
	}

2. 获取相较于当前时间的历史时间，如昨天、上周、上月、去年等

	public static String getYesterday(){
		Calendar c = Calendar.getInstance();
		c.add(Calendar.DATE, -1);
		SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy");
		return sdf.format(c.getTime());
	}
	
	public static String getLastMonth(){
		Calendar c = Calendar.getInstance();
		c.add(Calendar.MONTH, -1);
		SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy");
		return sdf.format(c.getTime());
	}
	
	public static String getLastYear(){
		Calendar c = Calendar.getInstance();
		c.add(Calendar.YEAR, -1);
		SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy");
		return sdf.format(c.getTime());
	}

3. 设置一个历史时间

	public static void setDate(){
		Calendar c = Calendar.getInstance();
		//note that the month start from 0
		c.set(2013, 0, 25, 14, 48, 20);
		SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");
		System.out.println(sdf.format(c.getTime()));
		
		String birthdate = "01/25/2012 13:11:12";
		Date date = new Date();
		try {
			date = sdf.parse(birthdate);
		} catch (ParseException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		c.setTime(date);
		System.out.println(sdf.format(c.getTime()));
	}

4. 时间与long型的对照关系，可以发现0对应的时间是 01/01/1970 08:00:00，以后每增加1s就要加上1000。这样借助于它我们也可以得到不同时间。

	public static void checkLongAndDate(){
		Calendar c = Calendar.getInstance();
		c.setTimeInMillis(0);
		SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");
		//output is 01/01/1970 08:00:00
		System.out.println(sdf.format(c.getTime()));
		c.setTimeInMillis(c.getTimeInMillis()+1000*60*60*24);
		System.out.println(sdf.format(c.getTime()));
	}