发现朋友做一个项目，然后涉及到年、月、日的加减。他居然自己定义了24小时，31天，12个月几个数组，然后自己写逻辑运算。首先不说自己写出来有没有BUG，至少肯定代码有点多，而且还要判断闰年。Java在很早的版本中已经提供了时间操作类，JDK1.1好像就有Calendar日期操作类，利用它，你随意运算，而且代码量非常小。
以下例子是建立在每次更改之后的时间，并不是每次都是当前时间。

		SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd HH:mm:ss");
		System.out.println("现在时间");
		Calendar calendar = Calendar.getInstance();
		System.out.println(sdf.format(new Date(calendar.getTimeInMillis())));
		int day = calendar.get(Calendar.DAY_OF_YEAR);
		// 30天前
		calendar.set(Calendar.DAY_OF_YEAR, day - 30);
		System.out.println("30天前");
		System.out.println(sdf.format(new Date(calendar.getTimeInMillis())));
		// 78小时前
		int hour = calendar.get(Calendar.HOUR_OF_DAY);
		calendar.set(Calendar.HOUR_OF_DAY, hour - 78);
		System.out.println("78小时前");
		System.out.println(sdf.format(new Date(calendar.getTimeInMillis())));
		int second = calendar.get(Calendar.SECOND);
		calendar.set(Calendar.SECOND, second - 5362);
		System.out.println("5362秒前");
		System.out.println(sdf.format(new Date(calendar.getTimeInMillis())));

时间解析：

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Calendar c = Calendar.getInstance();
		// 2015-11-8 0:59:54
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
		try {
			Date date = sdf.parse("2015-11-8 0:59:54");
			c.setTime(date);
			System.out.print(c.get(Calendar.YEAR));
			System.out.print(c.get(Calendar.MONTH) + 1);
			System.out.print(c.get(Calendar.DAY_OF_MONTH));
			System.out.print(c.get(Calendar.HOUR_OF_DAY));
			System.out.print(c.get(Calendar.MINUTE));
			System.out.print(c.get(Calendar.SECOND));
		} catch (ParseException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

 

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