Calculate XNPV

CalcXNPV.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CalculateXNPV
{
    class Calculate
    {
        IEnumerable<DateTime> dates;
        IEnumerable<int> payments;

        public Calculate(IEnumerable<DateTime> dates, IEnumerable<int> payments)
        {
            this.dates = dates;
            this.payments = payments;
        }

        public double CalcXnpv(int period, double rate)
        {
            if (period == 0)
            {
                return payments.ElementAt(0);
            }
            TimeSpan span = dates.ElementAt(period) - dates.ElementAt(0);
            return CalcXnpv(period - 1, rate) + (payments.ElementAt(period) / Math.Pow((1 + (rate/365)), span.Days));
        }

    }
}