Computes the net present value (NPV) of a non-periodic cash flow with constant discount rate. The rate parameter is the annual discount rate for a 365 day year. The values parameter is indexed by I and contains the cash flow amounts, with positive values indicating inflows (payments received) and negative values indicating outflows (payments made). The dates parameter must also be indexed by I and contains the date in which each payment is made. The date is expressed as the number of days since the date origin (e.g., Jan 4, 1904).