// In reference to https://stackoverflow.com/a/22054654/1537714

​

using System;

using Xunit;

​

public class DateDiffer

{

    public int CalculateWeekdays(DateTime startDate, DateTime endDate)

    {

        // Ignore time

        startDate = startDate.Date;

        endDate = endDate.Date;

​

        int numDays = 1 + Convert.ToInt32((endDate - startDate).TotalDays);

        int numSaturdays = (numDays + Convert.ToInt32(startDate.DayOfWeek)) / 7;

​

        return numDays - 2 * numSaturdays

            - (startDate.DayOfWeek == DayOfWeek.Sunday ? 1 : 0)

            + (endDate.DayOfWeek == DayOfWeek.Saturday ? 1 : 0);

    }

}

​

public class Program

{

    public static void Main(string[] args)

    {

        // Arrange

        // Starts on a Sunday, ends on a Tuesday

        var d1 = new DateTime(2016, 5, 1);

        var d2 = new DateTime(2016, 5, 31);

        // Starts on a Monday, ends on a Monday (and is a leap year)

        var d3 = new DateTime(2016, 2, 1);

        var d4 = new DateTime(2016, 2, 29);

        // Starts on a Tuesday, ends on a Thursday

        var d5 = new DateTime(2016, 3, 1);

        var d6 = new DateTime(2016, 3, 31);

        // Starts on a Wednesday, ends on a Thursday

        var d7 = new DateTime(2016, 6, 1);

        var d8 = new DateTime(2016, 6, 30);

        // Starts on a Thursday, ends on a Friday

        var d9 = new DateTime(2016, 9, 1);

        var d10 = new DateTime(2016, 9, 30);

        // Starts on a Friday, ends on a Sunday

        var d11 = new DateTime(2016, 7, 1);

        var d12 = new DateTime(2016, 7, 31);

        // Starts on a saturday, ends on a Monday

        var d13 = new DateTime(2016, 10, 1);

        var d14 = new DateTime(2016, 10, 31);

        // Example from original poster on SO

        var d15 = new DateTime(2009, 11, 30); 

        var d16 = new DateTime(2009, 12, 4); 

​

        // Act

        var differ = new DateDiffer();

        int numFromSunday = differ.CalculateWeekdays(d1, d2);

        int numFromMonday = differ.CalculateWeekdays(d3, d4);

        int numFromTuesday = differ.CalculateWeekdays(d5, d6);

        int numFromWednesday = differ.CalculateWeekdays(d7, d8);

        int numFromThursday = differ.CalculateWeekdays(d9, d10);

        int numFromFriday = differ.CalculateWeekdays(d11, d12);

        int numFromSaturday = differ.CalculateWeekdays(d13, d14);

        int exampleFromOp = differ.CalculateWeekdays(d15, d16);

​

        // Assert

        Assert.Equal(22, numFromSunday);

        Assert.Equal(21, numFromMonday);

        Assert.Equal(23, numFromTuesday);

        Assert.Equal(22, numFromWednesday);

        Assert.Equal(22, numFromThursday);

        Assert.Equal(21, numFromFriday);

        Assert.Equal(21, numFromSaturday);

        Assert.Equal(5, exampleFromOp);

        Console.WriteLine($"{nameof(numFromSunday)} = {numFromSunday}");

        Console.WriteLine($"{nameof(numFromMonday)} = {numFromMonday}");

        Console.WriteLine($"{nameof(numFromTuesday)} = {numFromTuesday}");

        Console.WriteLine($"{nameof(numFromWednesday)} = {numFromWednesday}");

        Console.WriteLine($"{nameof(numFromThursday)} = {numFromThursday}");

        Console.WriteLine($"{nameof(numFromFriday)} = {numFromFriday}");

        Console.WriteLine($"{nameof(numFromSaturday)} = {numFromSaturday}");

        Console.WriteLine($"{nameof(exampleFromOp)} = {exampleFromOp}");

    }

}