计算相对今天任意a天前的日期，a可正可负

	public static String calculateDateByNumber(int a) {
		String str = "";
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
		Date date = new Date();
		Calendar cal = Calendar.getInstance();
		cal.setTime(date);
		cal.add(Calendar.DATE, a);
		str = sdf.format(cal.getTime());
		return str;
	}

	// "2012-09-20"转换为java.util.Date类型2012-09-20
	public static Date stringToUtilDate(String str) {
		SimpleDateFormat simple = new SimpleDateFormat("yyyy-MM-dd");
		Date tempTime = null;
		try {
			tempTime = simple.parse(str.trim());
		} catch (ParseException e) {
			e.printStackTrace();
		}
		return tempTime;
	}

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

public class Testwmf {

	public static void main(String[] args) {
		int page=0;
		for (int i = page; i > -40; i--) {
			Date d = new Date();
			String time = calculateDateByNumber(i);
			System.out.println("=====i:"+i+"，时间："+time);
		}
	}
	public static String calculateDateByNumber(int a) {
		String str = "";
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
		Date date = new Date();
		Calendar cal = Calendar.getInstance();
		cal.setTime(date);
		cal.add(Calendar.DATE, a);
		str = sdf.format(cal.getTime());
		return str;
	}
}