public String format(Date then)
   {
      if (then == null)
         then = now();

      Duration d = approximateDuration(then);
      return format(d);
   }
@Test
   public void testWeeksAgo() throws Exception
   {
      PrettyTime t = new PrettyTime(now);
      Assert.assertEquals("3 weeks ago", t.format(now.minusWeeks(3)));
   }