code: private static DateFormat getDateFormat() {
  		if (IS_JAVA7) {
  			SoftReference<DateFormat> softReference = THREAD_LOCAL_DF.get();
  			if (softReference != null) {
  				DateFormat dateFormat = softReference.get();
  				if (dateFormat != null) {
  					return dateFormat;
  				}
  			}
  			DateFormat result = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSXXX", Locale.US);
  			softReference = new SoftReference<DateFormat>(result);
  			THREAD_LOCAL_DF.set(softReference);
  			return result;
  		} else {
  			throw new RuntimeException("Error parsing XES log. This method should not be called unless running on Java 7!");
  		}
  	}
docs: Returns a DateFormat for each calling thread, using {@link ThreadLocal}.
  
  @return a DateFormat that is safe to use in multi-threaded environments