Rename the files with a rule of dates.

Question

I have written a Photo Renamer code which renames the photos according to their date taken. After the date I added a sequence number like 001, 002, 003,.... Finally I got the 2013.07.12.001.JPG name format. But there are photos on different dates and it is nonsense sequence number's follow the other days sequence. Every date must have its own sequence, not other day's.
You can see the problem at my photo.

How can I edit the loop part of my code in order to get such rename:

2013.07.12.001.JPG
2013.07.12.002.JPG
2013.07.12.003.JPG
2013.07.12.004.JPG
2013.07.12.005.JPG
2013.08.27.001.JPG
2013.07.11.001.JPG
2013.07.11.002.JPG
2013.07.11.003.JPG
2013.07.11.004.JPG
2013.07.11.005.JPG

My codes are here:
============================================
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Drawing.Imaging; //YENI
using System.IO; //YENI

namespace PhotoRenamer2
{
  public partial class Form1 : Form
  {
  public Form1()
  {
  InitializeComponent();
  }

  private void button1_Click(object sender, EventArgs e)
  {
  DialogResult result = folderBrowserDialog1.ShowDialog();
  string pathname = @folderBrowserDialog1.SelectedPath;
  label2.Text = pathname;
  FileInfo[] files = new DirectoryInfo(pathname).GetFiles("*.JPG");
  for (int i = 0; i < files.Length; i++)
  {
  string DateAndTimePictureTaken = string.Empty;
  Image image = new Bitmap(@files[i].FullName);
  PropertyItem[] propItems = image.PropertyItems;
  foreach (PropertyItem propItem in propItems)
  {
  if (propItem.Id == 0x0132)
  {
  DateAndTimePictureTaken = (new System.Text.ASCIIEncoding()).GetString(propItem.Value);
  image.Dispose();
  string Year = DateAndTimePictureTaken.Substring(0, 4);
  string Month = DateAndTimePictureTaken.Substring(5, 2);
  string Day = DateAndTimePictureTaken.Substring(8, 2);
  string PhotoNumber = (i+1).ToString().PadLeft(3, '0');
  string newFile = String.Format("{0}.{1}.{2}.{3}.{4}", Year, Month, Day, PhotoNumber, "JPG");
  string fullNewPath = files[i].DirectoryName;
  string newFileWithPath = Path.Combine(fullNewPath, newFile);
  label2.Text = files[i].FullName;
  label3.Text = newFileWithPath;
  File.Move(files[i].FullName, newFileWithPath);
  break;
  }
  }
  }
  }

  }
}
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