code: public static Days days(int days) {
          switch (days) {
              case 0:
                  return ZERO;
              case 1:
                  return ONE;
              case 2:
                  return TWO;
              case 3:
                  return THREE;
              case 4:
                  return FOUR;
              case 5:
                  return FIVE;
              case 6:
                  return SIX;
              case 7:
                  return SEVEN;
              case Integer.MAX_VALUE:
                  return MAX_VALUE;
              case Integer.MIN_VALUE:
                  return MIN_VALUE;
              default:
                  return new Days(days);
          }
      }
docs: Obtains an instance of <code>Days</code> that may be cached.
  <code>Days</code> is immutable, so instances can be cached and shared.
  This factory method provides access to shared instances.
  
  @param days  the number of days to obtain an instance for
  @return the instance of Days