title: Call to method of static java.text.DateFormat not advisable?
body: <p>I am receiving a <em>Find Bugs</em> error - <strong>call to method of static java.text.DateFormat</strong> and 
  I don't know the reason why it's not good / advisable to be doing these things below.</p>
  
  <pre><code>private static final Date TODAY = Calendar.getInstance().getTime();
  private static final DateFormat yymmdd = new SimpleDateFormat("yyMMdd"); 
  
  private String fileName = "file_" + yymmdd.format(TODAY);
  </code></pre>