Java计算两个日期相差年月日的方法

package com.main;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class Test01 {
  private static Calendar calS=Calendar.getInstance();   
    private static Pattern   p   =   Pattern.compile("\\d{4}-\\d{2}-\\d{2}");//定义整则表达式   
   
    /**  
     * 计算剩余时间  
     * @param startDateStr  
     * @param endDateStr  
     * @return  
     */   
    public static String remainDateToString(String startDateStr, String endDateStr){   
        java.util.Date startDate = null;   
        java.util.Date endDate= null;   
        try {   
            startDate = new SimpleDateFormat("yyyy-MM-dd").parse(startDateStr);   
            endDate = new SimpleDateFormat("yyyy-MM-dd").parse(endDateStr);   
        } catch (Exception e) {   
            e.printStackTrace();   
            return "";   
        }   
        calS.setTime(startDate);   
        int startY = calS.get(Calendar.YEAR);   
        int startM = calS.get(Calendar.MONTH);   
        int startD = calS.get(Calendar.DATE);   
        int startDayOfMonth = calS.getActualMaximum(Calendar.DAY_OF_MONTH);