code: Duration addDays(final int amount) {
          Duration duration;
          if (amount < 0) {
              duration = getDatatypeFactory().newDuration(false, 0, 0, Math.abs(amount), 0, 0, 0);
          } else {
              duration = getDatatypeFactory().newDuration(true, 0, 0, amount, 0, 0, 0);
          }
          return duration;
      }
docs: Create a Duration of x days.
  
  @param amount The number of days. Use negative numbers to substract days
  @return a Duration