title: Windows: How to determine if a file has been modified since a given date
body: <p>I have a utility which goes through a processes a set of files in a directory - the process is relatively slow (and there are a lot of files) and so I've tried to optimise the process by only processes files that have a "last modified" later than the last processing date.</p>
  
  <p>Usually this works well however I've found that as copying a file doesn't change the last modified date, and so there are various scenarios involving copying files in which certain files that have changed are skipped by the process, for example:</p>
  
  <ol>
  <li>The user processes the directory at 9:00.</li>
  <li>A file is then copied from this directory and modified so that it has a last modified date of 9:30</li>
  <li>The directory is then processed again at 10:00</li>
  <li>The modified file is then copied back into the directory at 10:30</li>
  <li>Finally the directory is processed again at 11:00</li>
  </ol>
  
  <p>As the modified date of the given file is 9:30, and the directory was last processed at 10:00 the file is skipped when it shouldn't be.</p>
  
  <p>Unfortunately the above tends to happen far too often in certain situations (such as in a collaborative environment with source control etc...).  Clearly my logic is flawed - what I really need is a "last modified or copied" date. does such a thing exist?</p>
  
  <p>Failing that, is there another way to quickly determine with reasonable reliability if a given file has changed?</p>