FeeCalculator.cs

public class FeeCalculator
{
    public const decimal DailyFee = 50.0m;

    public decimal FeeFor(DateTime startTime)
    {
        DateTime now = ApplicationTime.Current;

        if (startTime > now)
        {
            throw new ArgumentException("Cannot calculate fee for future time");
        }

        int daysCount = (int)(now - startTime).TotalDays;

        return DailyFee * daysCount;
    }
}

FeeCalculatorTests.cs

public class FeeCalculatorTests : IDisposable
{
    private FeeCalculator _calculator;

    public FeeCalculatorTests()
    {
        ApplicationTime._replaceCurrentTimeLogic(() => new DateTime(2011, 03, 14, 13, 00, 00));

        _calculator = new FeeCalculator();
    }

    public void Dispose()
    {
        ApplicationTime._revertToDefaultLogic();
    }

    [Fact]
    public void returns_multiplication_of_days()
    {
        decimal fee = _calculator.FeeFor(new DateTime(2011, 03, 10));

        Assert.Equal(200m, fee);
    }

    [Fact]
    public void returns_0_if_the_same_day()
    {
        decimal fee = _calculator.FeeFor(new DateTime(2011, 03, 14, 10, 13, 00));

        Assert.Equal(0.0m, fee);
    }

    [Fact]
    public void throws_if_future_time()
    {
        Assert.Throws<ArgumentException>(
            () => _calculator.FeeFor(new DateTime(2011, 05, 03))
        );
    }
}