code: public Duration approximateDuration(Date then)
     {
        if (then == null)
           then = now();
  
        Date ref = reference;
        if (null == ref)
           ref = now();
  
        long difference = then.getTime() - ref.getTime();
        return calculateDuration(difference);
     }
docs: Calculate the approximate {@link Duration} between the reference {@link Date} and given {@link Date}. If the given
  {@link Date} is <code>null</code>, the current value of {@link System#currentTimeMillis()} will be used instead.
  <p>
  See {@code PrettyTime#getReference()}.