java 日期函数

得到过去的时间:
example one:

private Date getDateTime(){
	Calendar calendar = Calendar.getInstance();
	calendar.set(2011, Calendar.DECEMBER, 1, 23, 0, 0);
	return calendar.getTime();
}

example two:

String.valueOf(new Date().getTime() - 24 * 60 * 60 * 1000)

转换日期成String:

private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

public static String covertDate(Date d) {
	String ret = "";
	if (d != null) {
		ret = sdf.format(d);
	}
	return ret;
}