java poi excel table imports java date type has changed how to solve

in the Excel table

java

Why did this date change
Code:
package data_sorting;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;
import java.io.FileInputStream;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.List;
import java.util.Scanner;

import javax.swing.JButton;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;

import org.apache.poi.xssf.usermodel.XSSFCell;
import org.apache.poi.xssf.usermodel.XSSFRow;
import org.apache.poi.xssf.usermodel.XSSFSheet;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;

public class Test5 {

static List<String> hbvaluelist = null;
public static void main(String[] args) {
    JFrame frame = new JFrame("");
    JButton button = new JButton("");
    JButton button2 = new JButton("");
    final JTextField tf = new JTextField();
    final JTextField tf2 = new JTextField();
    final JTextField tf3 = new JTextField();
    final JTextArea tf4 = new JTextArea();
    final JTextField tf5 = new JTextField("yyyy/MM/dd");
    final JTextField tf6 = new JTextField();
    JScrollPane jsp = new JScrollPane(tf4);
    jsp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
    JLabel jl1 = new JLabel(":");
    JLabel jl2 = new JLabel(":");
    JLabel jl3 = new JLabel(":");
    JLabel jl4 = new JLabel(":");
    JLabel jl5 = new JLabel("*:yyyy :MM :dd :2000/10/20 :yyyy/MM/dd");
    JLabel jl6 = new JLabel(":");
    JLabel jl7 = new JLabel("* ");
    frame.setLayout(null);
    frame.setSize(800, 700);
    jl1.setBounds(110, 0, 160, 30);
    jl2.setBounds(20, 35, 50, 30);
    jl3.setBounds(20, 70, 50, 30);
    jl4.setBounds(10, 105, 90, 30);
    jl5.setBounds(200, 105, 400, 30);
    jl6.setBounds(150, 70, 50, 30);
    jl7.setBounds(250, 70, 200, 30);
    button.setBounds(0, 0, 100, 30);
    button2.setBounds(0, 140, 100, 30);
    tf.setBounds(160, 0, 500, 30);
    tf2.setBounds(90, 35, 50, 30);
    tf3.setBounds(90, 70, 50, 30);
    tf5.setBounds(100, 105, 80, 30);
    jsp.setBounds(10, 175, 770, 400);
    tf6.setBounds(200, 70, 50, 30);
    frame.add(button);
    frame.add(button2);
    frame.setVisible(true);
    frame.add(tf);
    frame.add(tf2);
    frame.add(tf3);
    frame.add(jsp);
    frame.add(tf5);
    frame.add(tf6);
    frame.add(jl1);
    frame.add(jl2);
    frame.add(jl3);
    frame.add(jl4);
    frame.add(jl5);
    frame.add(jl6);
    frame.add(jl7);
    button.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            // 
            JFileChooser chooser = new JFileChooser(); // 
            chooser.setMultiSelectionEnabled(false); // 
            int returnVal = chooser.showOpenDialog(button); // 
            
            if (returnVal == JFileChooser.APPROVE_OPTION) { // 

                String filepath = chooser.getSelectedFile().getAbsolutePath(); // 
                tf.setText(filepath);
                button2.addActionListener(new ActionListener() {
                    
                    @Override
                    public void actionPerformed(ActionEvent e) {
                        tf4.setText( " ");
                        String url = tf.getText();
                        String sheet = tf2.getText();
                        String num = tf3.getText();
                        String format = tf5.getText();
                        String num2 = tf6.getText();
                        try {
                            List<String> hbdatelist = showExcel(url, sheet, num, format,num2);
                            for (int i = 0; i < hbdatelist.size(); iPP) {
                                
                                tf4.append(hbdatelist.get(i) + "\t" + hbvaluelist.get(i)+"\n");
                            }
                        } catch (Exception e1) {
                            e1.printStackTrace();
                        }
                        
                    }
                });
                System.out.println(filepath);
                System.out.println("You chose to open this file: " + chooser.getSelectedFile().getName()); // 
                
            }
        }
    });
}
public static List<String> showExcel(String url,String sheet,String num,String format,String num2) throws Exception {
    List<String> datev = new ArrayList<String>();
    List<String> value = new ArrayList<String>();
    List<String> newdatevlist = new ArrayList<String>();
    List<String> hbdatelist = new ArrayList<String>();
    hbvaluelist = new ArrayList<String>();
    
    SimpleDateFormat simp = new SimpleDateFormat(format);
    //SimpleDateFormat simp = new SimpleDateFormat("yyyy-MM-dd");
    XSSFWorkbook hssfWorkbook = new XSSFWorkbook(new FileInputStream(new File(url)));
    // 
    int sh = Integer.parseInt(sheet)-1;
    int nu = Integer.parseInt(num)-1;
    int nu2 = Integer.parseInt(num2)-1;
    XSSFSheet sheetAt = hssfWorkbook.getSheetAt(sh);
    if (sheetAt == null) {
        
    }
    String starttime = sheetAt.getRow(nu2).getCell(nu).toString();
    String endtime = sheetAt.getRow(sheetAt.getLastRowNum()).getCell(nu).toString();
    System.err.println(starttime);
    System.err.println(endtime);
    Date startdate = simp.parse(starttime);
    Date enddate = simp.parse(endtime);
    List<Date> dateList = findDates(startdate, enddate);
    for (Date date : dateList) {
        newdatevlist.add(simp.format(date));

    }

    // 
    for (int i = nu2; i < sheetAt.getLastRowNum() + 1; iPP) {
        XSSFRow row = sheetAt.getRow(i);
        if (row == null) {
            continue;
        }
        // 
        XSSFCell cell1 = row.getCell(nu);
        XSSFCell cell2 = row.getCell(nu-8);
        datev.add(cell1.toString().substring(0, 10));
        value.add(cell2.toString());

    }
    int count = 0;
    for (int i = 0; i < newdatevlist.size() - 1; iPP) {
        if (newdatevlist.get(i).equals(datev.get(count))) {
            //hbdatelist.add(newdatevlist.get(i));
            hbdatelist.add(newdatevlist.get(i));
            hbvaluelist.add(value.get(count));
            countPP;
        } else {
            //hbdatelist.add(newdatevlist.get(i));
            hbdatelist.add(newdatevlist.get(i));
            hbvaluelist.add(value.get(count - 1));
        }

    }
    hbdatelist.add(newdatevlist.get(newdatevlist.size() - 1));
    hbvaluelist.add(value.get(value.size() - 1));
    return hbdatelist;
    
}

// JAVA
public static List<Date> findDates(Date dStart, Date dEnd) {
    Calendar cStart = Calendar.getInstance();
    cStart.setTime(dStart);

    List<Date> dateList = new ArrayList<Date>();
    // 
    dateList.add(dStart);
    // 
    while (dEnd.after(cStart.getTime())) {
        // 
        cStart.add(Calendar.DAY_OF_MONTH, 1);
        dateList.add(cStart.getTime());
    }
    return dateList;
}

}