一、充分利用SimpleDateFormat

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

public class DateTestUtil {

    public static void main(String[] args) throws Exception {
        SimpleDateFormat sdf=new SimpleDateFormat("yyyyMMdd");
        String str="20110823";
        Date dt=sdf.parse(str);
        Calendar rightNow = Calendar.getInstance();
        rightNow.setTime(dt);
        rightNow.add(Calendar.YEAR,-1);//日期减1年
        rightNow.add(Calendar.MONTH,3);//日期加3个月
        rightNow.add(Calendar.DAY_OF_YEAR,10);//日期加10天
        Date dt1=rightNow.getTime();
        String reStr = sdf.format(dt1);
        System.out.println(reStr);
    }
}

注：在Calendar对象的add方法中，第二个参数为正数表示“加”，负数表示“减”。

二、java date 日期加减天数

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

public class DateTest { 

  public static void main(String[] arg){ 
    Date now = new Date();    

    addAndSubtractDaysByGetTime(now,-5); 
    addAndSubtractDaysByGetTime(now,5); 
    addAndSubtractDaysByCalendar(now,-5); 
    addAndSubtractDaysByCalendar(now,5); 
  } 

   public static Date addAndSubtractDaysByGetTime(Date dateTime/*待处理的日期*/,int n/*加减天数*/){ 

     //日期格式 
     SimpleDateFormat df=new SimpleDateFormat("yyyy-MM-dd");  
     SimpleDateFormat dd=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");     

     System.out.println(df.format(new Date(dateTime.getTime() + n * 24 * 60 * 60 * 1000L))); 
     //System.out.println(dd.format(new Date(dateTime.getTime() + n * 24 * 60 * 60 * 1000L))); 
     //注意这里一定要转换成Long类型，要不n超过25时会出现范围溢出，从而得不到想要的日期值 
     return new Date(dateTime.getTime() + n * 24 * 60 * 60 * 1000L); 
   } 

   public static Date addAndSubtractDaysByCalendar(Date dateTime/*待处理的日期*/,int n/*加减天数*/){ 

     //日期格式 
     SimpleDateFormat df=new SimpleDateFormat("yyyy-MM-dd");  
     SimpleDateFormat dd=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  

     java.util.Calendar calstart = java.util.Calendar.getInstance(); 
       calstart.setTime(dateTime); 

     calstart.add(java.util.Calendar.DAY_OF_WEEK, n);  

     System.out.println(df.format(calstart.getTime())); 
     //System.out.println(dd.format(calstart.getTime())); 
     return calstart.getTime(); 
   } 

}

不推荐这个，代码不够简洁，看着不爽。

三、网上找的别人的，太长，没看，粘过来先

package com.data.utils;

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

public class DateFormat {

 /**
  * 日期减几年
  */
 public static String dateMinusYear(String str) throws Exception {

  SimpleDateFormat sdf = new SimpleDateFormat("yyyyMM");
  Date dt = sdf.parse(str);
  Calendar rightNow = Calendar.getInstance();
  rightNow.setTime(dt);
  rightNow.add(Calendar.YEAR, -1);// 日期减1年
  Date dt1 = rightNow.getTime();
  String reStr = sdf.format(dt1);
  return reStr;
 }

 /**
  * 日期加几年
  */
 public static String dateAddYear(String str) throws Exception {

  SimpleDateFormat sdf = new SimpleDateFormat("yyyyMM");
  Date dt = sdf.parse(str);
  Calendar rightNow = Calendar.getInstance();
  rightNow.setTime(dt);
  rightNow.add(Calendar.YEAR, 1);// 日期加1年
  Date dt1 = rightNow.getTime();
  String reStr = sdf.format(dt1);
  return reStr;
 }

 /**
  * 日期减几月
  */
 public static String dateMinusMonth(String str) throws Exception {

  SimpleDateFormat sdf = new SimpleDateFormat("yyyyMM");
  Date dt = sdf.parse(str);//将字符串生成Date
  Calendar rightNow = Calendar.getInstance();
  rightNow.setTime(dt);//使用给定的 Date 设置此 Calendar 的时间。 
  rightNow.add(Calendar.MONTH, -1);// 日期减1个月
  Date dt1 = rightNow.getTime();//返回一个表示此 Calendar 时间值的 Date 对象。
  String reStr = sdf.format(dt1);//将给定的 Date 格式化为日期/时间字符串，并将结果添加到给定的 StringBuffer。
  return reStr;
 }

 /**
  * 日期加几月
  */
 public static String dateAddMonth(String str) throws Exception {

  SimpleDateFormat sdf = new SimpleDateFormat("yyyyMM");
  Date dt = sdf.parse(str);
  Calendar rightNow = Calendar.getInstance();
  rightNow.setTime(dt);
  rightNow.add(Calendar.MONTH, 1);// 日期加3个月
  // rightNow.add(Calendar.DAY_OF_YEAR,10);//日期加10天
  Date dt1 = rightNow.getTime();
  String reStr = sdf.format(dt1);
  return reStr;
 }

 /**
  * 获取当前年月的第一个月的str
  * @param str
  *   201505
  * @return 201501
  * @throws Exception
  */
 public static String dateOneMonth(String str) {

  str = str.substring(0, str.length() - 2);
  str = str + "01";
  return str;
 }

 /**
  * 算出所选月份距离一月份有几个月。
  * @param str 201509
  * @return 9
  */
 public static int dateDistanceMonth(String str) {

  int i = Integer.parseInt(str);
  int j = Integer.parseInt(DateFormat.dateOneMonth(str));
  System.out.println(i - j);
  return i - j + 1;
 }

 /**
  * 获取两个时间的时间差，精确到毫秒
  * @param str
  * @return
  */
 public static String TimeDifference(long start, long end) {

  long between = end - start;
  long day = between / (24 * 60 * 60 * 1000);
  long hour = (between / (60 * 60 * 1000) - day * 24);
  long min = ((between / (60 * 1000)) - day * 24 * 60 - hour * 60);
  long s = (between / 1000 - day * 24 * 60 * 60 - hour * 60 * 60 - min * 60);
  long ms = (between - day * 24 * 60 * 60 * 1000 - hour * 60 * 60 * 1000
    - min * 60 * 1000 - s * 1000);
  String timeDifference = day + "天" + hour + "小时" + min + "分" + s + "秒" + ms
    + "毫秒";
  return timeDifference;
 }
}

 /**
  * 获取24小时、一周、一个月的起始时间
  * 
  * @param timeInterval
  *   : DAY_TIME_INTERVAL WEEK_TIME_INTERVAL MONTH_TIME_INTERVAL
  * @return "yyyy-mm-dd hh:mm:ss"
  */
 public static String getStartTime(int timeInterval) {
  Calendar cal = Calendar.getInstance();
  SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
  if (DAY_TIME_INTERVAL == timeInterval) {// 获取24小时的起始时间
   cal.set(Calendar.HOUR_OF_DAY, 0);
   cal.set(Calendar.MINUTE, 0);
   cal.set(Calendar.SECOND, 0);
   String startTime = sdf.format(cal.getTime());
   return startTime;
  } else if (WEEK_TIME_INTERVAL == timeInterval) {
   int weekday = cal.get(Calendar.DAY_OF_WEEK) - 1;
   cal.add(Calendar.DATE, -weekday);
   cal.set(Calendar.HOUR_OF_DAY, 0);
   cal.set(Calendar.MINUTE, 0);
   cal.set(Calendar.SECOND, 0);
   String startTime = sdf.format(cal.getTime());
   return startTime;
  } else if (MONTH_TIME_INTERVAL == timeInterval) {
   int dayofmonthMin = cal.getActualMinimum(Calendar.DAY_OF_MONTH);
   // c.add(Calendar.DATE, -dayofmonth);
   cal.set(Calendar.DATE, dayofmonthMin);
   cal.set(Calendar.HOUR_OF_DAY, 0);
   cal.set(Calendar.MINUTE, 0);
   cal.set(Calendar.SECOND, 0);
   String startTime = sdf.format(cal.getTime());
   return startTime;
  }
  return null;
 }

 /**
  * 获取24小时、一周、一个月的结束时间
  * 
  * @param timeInterval
  *   : DAY_TIME_INTERVAL WEEK_TIME_INTERVAL MONTH_TIME_INTERVAL
  * @return "yyyy-mm-dd hh:mm:ss"
  */
 public static String getEndTime(int timeInterval) {
  Calendar cal = Calendar.getInstance();
  cal.setTimeZone(TimeZone.getTimeZone("GMT+8"));
  SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
  if (DAY_TIME_INTERVAL == timeInterval) {
   cal.set(Calendar.HOUR_OF_DAY, 23);
   cal.set(12, 59);
   cal.set(13, 59);
   long date = cal.getTimeInMillis();
   String endTime = sdf.format(new Date(date));
   return endTime;
  } else if (WEEK_TIME_INTERVAL == timeInterval) {
   int weekday = cal.get(Calendar.DAY_OF_WEEK) - 1;
   cal.add(Calendar.DATE, -weekday);
   cal.add(Calendar.DATE, 6);
   cal.set(Calendar.HOUR_OF_DAY, 23);
   cal.set(12, 59);
   cal.set(13, 59);
   long date = cal.getTimeInMillis();
   String endTime = sdf.format(new Date(date));
   return endTime;
  } else if (MONTH_TIME_INTERVAL == timeInterval) {
   int dayOfMonthMax = cal.getActualMaximum(Calendar.DAY_OF_MONTH);
   cal.set(Calendar.DATE, dayOfMonthMax);
   cal.set(Calendar.HOUR_OF_DAY, 23);
   cal.set(Calendar.MINUTE, 59);
   cal.set(Calendar.SECOND, 59);
   String endTime = sdf.format(cal.getTime());
   return endTime;
  }
  return null;
 }
 /**
  * 判断dateStr是否在start和end中间，start和end都可以为null yyyyMMddHHmmss或者yyyyMMdd格式
  * 
  * @author you.xu
  * @date 2015年8月19日下午3:11:46
  * @param dateStr
  * @param start
  * @param end
  * @return
  */
 public static boolean checkDateVal(String dateStr, String start, String end) {
  boolean isDateRight = false;
  Date date = null;
  Date startDate = null;
  Date endDate = null;
  SimpleDateFormat sdf = null;
  // 判断日期格式
  if (14 == dateStr.length()) {
   sdf = new SimpleDateFormat("yyyyMMddHHmmss");
  } else if (8 == dateStr.length()) {
   sdf = new SimpleDateFormat("yyyyMMdd");
  } else
   return false;

  try {
   // 更改判断日期格式
   date = sdf.parse(dateStr);
  } catch (ParseException e) {
   log.error(e, e);
  }

  if ((start == null) && (end != null)) {
   try {
    endDate = sdf.parse(end);
   } catch (ParseException ex1) {
    log.error(ex1, ex1);
   }
   if ((date != null) && (endDate != null))// Check parameters for
   {
    if (date.compareTo(endDate) <= 0)
     isDateRight = true;
   }
  } else if ((start != null) && (end == null)) {
   try {
    startDate = sdf.parse(start);
   } catch (ParseException ex1) {
    log.error(ex1, ex1);
   }
   if ((date != null) && (startDate != null)) {
    if (date.compareTo(startDate) >= 0)
     isDateRight = true;
   }
  } else if ((start != null) && (end != null)) {
   try {
    startDate = sdf.parse(start);
    endDate = sdf.parse(end);
   } catch (ParseException ex2) {
    System.out.println(ex2.toString());
   }
   if ((startDate != null) && (date != null) && (endDate != null)) {
    if ((date.compareTo(startDate) >= 0)
      && (date.compareTo(endDate) <= 0))
     isDateRight = true;
   }
  }
  return isDateRight;
 }

 /**
  * 判断dateStr是否在start和end中间，start和end都可以为null long形格式
  * 
  * @author you.xu
  * @date 2015年8月19日下午3:12:35
  * @param dateStr
  * @param start
  * @param end
  * @return
  */
 public static boolean checkDateV(String dateStr, String start, String end) {
  boolean isDateRight = false;
  long date = -1;
  long fromDate = -1;
  long toDate = -1;

  date = java.lang.Long.parseLong(dateStr);

  if ((start == null) && (end == null)) {
   isDateRight = true;
  } else if ((start == null) && (end != null)) {
   try {
    toDate = java.lang.Long.parseLong(end);
   } catch (NumberFormatException nfe) {
    log.error(nfe, nfe);
   }
   if (date <= toDate) {
    isDateRight = true;
   }
  } else if ((start != null) && (end == null)) {
   try {
    fromDate = java.lang.Long.parseLong(start);
   } catch (NumberFormatException nfe) {
    log.error(nfe, nfe);
   }

   if (date >= fromDate) {
    isDateRight = true;
   }
  } else if ((start != null) && (end != null)) {
   try {
    toDate = java.lang.Long.parseLong(end);
    fromDate = java.lang.Long.parseLong(start);
   } catch (NumberFormatException nfe) {
    log.error(nfe, nfe);
   }

   if ((date <= toDate) && (date >= fromDate)) {
    isDateRight = true;
   }
  }
  return isDateRight;
 }

四、自己的方法的，简单说下需求，由于数据库里的存储类型是 “2017-07-04 12:00:00”，要按当前时间向前检索传进来的天数（noTradeDays），利用java.util的GregorianCalendar，结合SimpleDateFormat，完成当前时间逆推的需求

        /**
         * add by lee 2017/7/4 
         * 连续n天无交易
         */
        if (!StringUtils.isEmpty(noTradeDays)) {
            **SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            GregorianCalendar gc=new GregorianCalendar(); 
            gc.setTime(new Date());
            gc.add(5, -Integer.parseInt(noTradeDays));**
            String cardSql="select posid from card where usetime between "+"'"+sdf.format(gc.getTime())+"'"+" and "+"'"+sdf.format(new Date())+"'"+" group by posid";
            List<Card> cards = CardService.cardDAO.executeQuery(new Query(cardSql));
            if (cards.size()>0) {
                StringBuffer sb=new StringBuffer();
                for (Card posId : cards) {
                    if (!StringUtils.isEmpty(posId.getPosid())) {
                        sb.append("'").append(posId.getPosid()).append("'").append(",");
                    }
                }
                if (sb.length()>0) {
                    sql_add +=" and posid not in ("+sb.substring(0, sb.length()-1).toString()+")";
                }
            }
        }

恩，就是这样子了，总之就是灵活运用java.util里关于日期的相关类。