Call to method of static java.text.DateFormat not advisable?

I am receiving a Find Bugs error - call to method of static java.text.DateFormat and I don't know the reason why it's not good / advisable to be doing these things below.

private static final Date TODAY = Calendar.getInstance().getTime();
private static final DateFormat yymmdd = new SimpleDateFormat("yyMMdd"); 

private String fileName = "file_" + yymmdd.format(TODAY);