code: public double computeValue(double srcValue)
      {
          double dateOtherDate = 0;
          double dateThisDate = 0;
          if (m_fldOtherDate != null)
              dateOtherDate = m_fldOtherDate.getValue();
          else
              dateOtherDate = DateField.todaysDate();
          dateThisDate = ((DateTimeField)this.getOwner()).getValue();
          double tsDifference = (Math.floor((dateThisDate - dateOtherDate + 0.5)) / DBConstants.KMS_IN_A_DAY);  // Rounded number of days
          return tsDifference;    // Difference between this date and target
      }
docs: Compute the target value.
  @param srcValue The value of this listener's owner.
  @return The value to set the target field to.