title: Why md5 always changes on a certain file?
body: <p>I have this task that needs investigation as to why the md5 value of a file keeps changing.</p>
  
  <p>Example:</p>
  
  <ol>
  <li><p>I need to generate the diagnostic file of a certain machine.</p></li>
  <li><p>After generating the file, it produces a .zip file, say, Diag.zip which contains all the information/files of that certain machine.</p></li>
  <li><p>Inside Diag.zip file contain a .xls, say, Data.xls which contains all the summary of all files in that certain machine, includes, the directory of the file, file version, file size, create time and md5.</p></li>
  <li><p>Then save all the information of Data.xls in database.</p></li>
  </ol>
  
  <p>After a day or so, do it again back in Step 1-4.</p>
  
  <p>Then when I queried all the save data of Data.xls in the database in a 2 weeks range, and it shows that almost all files in that certain machine have its md5 value changed.</p>
  
  <p>The question is: Why is it that md5 value always changed every time I generated a new diagnostic files?</p>