计算相对今日任意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2012-09-20public 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;}}