using System;

using System.Collections.Generic;

​

public static class TestHelper

{

    public static IEnumerable<TestData> InitTestData()

    {

        var result = new List<TestData>

        {

            // First parameter: Initial Date (with time)

            // Second parameter: due days to adding

            // Third parameter: expected Due Date (with time)

            new TestData(DateTime.Parse("04/09/2022 04:57:00 PM"), 6, DateTime.Parse("04/18/2022 06:00:00 PM")),

            new TestData(DateTime.Parse("04/07/2022 04:57:00 PM"), 4, DateTime.Parse("04/12/2022 06:00:00 PM")),

            new TestData(DateTime.Parse("04/07/2022 08:57:00 PM"), 4, DateTime.Parse("04/13/2022 06:00:00 PM")),

            new TestData(DateTime.Parse("04/07/2022 04:57:00 PM"), 8, DateTime.Parse("04/18/2022 06:00:00 PM")),

            new TestData(DateTime.Parse("04/08/2022 05:57:00 PM"), 6, DateTime.Parse("04/18/2022 06:00:00 PM"))

        };

​

        return result;

    }

}

public class Program

{

    public static void Main()

    {

        var testData = TestHelper.InitTestData();

        // Calculations

        var dateCalculator = new DateCalculator();

        foreach (var data in testData)

        {

            var dueDate = dateCalculator.AddBusinessDays(data.InitialDate, data.DueDays);

            // Print results

            Console.Write("Initial date: ");

            Console.WriteLine(data.InitialDate.ToString("ddd, MM/dd/yyyy HH:mm:ss"));

            Console.Write("Due date: ");

            Console.WriteLine(dueDate.ToString("ddd, MM/dd/yyyy HH:mm:ss"));

            Console.Write("Expected Due date: ");

            Console.WriteLine(data.ExpectedDueDate.ToString("ddd, MM/dd/yyyy HH:mm:ss"));

            Console.Write("Calculations correct: ");

            Console.WriteLine(dueDate == data.ExpectedDueDate);

            Console.WriteLine();

        }

    }

}

​

public class DateCalculator

{

    private static readonly TimeSpan START_WORKING_HOUR = new TimeSpan(9, 0, 0); // 09:00

    private static readonly TimeSpan END_WORKING_HOUR = new TimeSpan(18, 0, 0); // 18:00

    private static readonly TimeSpan END_WORKING_HOUR_FRIDAY = new TimeSpan(17, 0, 0); // 17:00

​

    public DateTime AddBusinessDays(DateTime sourceTime, int addDays)

    {

        if( addDays == 0 ) {

            return sourceTime;

        }

​

        var resultDate = sourceTime;

​

        var workingDay = isWorkingDay(resultDate);

​

        // If we start calculate from non-working day or on non-working time

        // Add days if starting day is non-working day or working day already ended

        if (!workingDay) {

            do {

                resultDate = resultDate.AddDays(1);

                workingDay = isWorkingDay( resultDate );

            } while (!workingDay);

​

                /* 

                 * If first day is non-working (holiday) there actually no "working hours".

                 * Since working hour will effect further calculation, first working day morning time will be set as reference point.

                 */

            resultDate = resetToMorningTime(resultDate);

        }

​

            /*

             * If creation/warning hour is after (or equal) to end of work hour then task should be finished until end of next work day.

             * If creation/warning hour is before end of work hour then task should finished until end of this work day.

             * 

             * Since end of work hour may differ between different week days (e.g. Saturday vs Monday)

             * Decision if task should be done until end of working day should be made before before applying days from period. 

             */

        if (isBeforeEndOfWorkingTime(resultDate)) {

            addDays--;

        }

​

        while( addDays > 0 ) {

            resultDate = resultDate.AddDays(1);

            if(isWorkingDay(resultDate) ) {

                addDays--;

            }

        }

​

        // task expected to be done until end of day

        resultDate = resetToEveningTime(resultDate);

​

        return resultDate;

    }

​

    private bool isWorkingDay(DateTime date)

    {

        switch (date.DayOfWeek)

        {

            case DayOfWeek.Monday:

            case DayOfWeek.Tuesday:

            case DayOfWeek.Wednesday:

            case DayOfWeek.Thursday:

            case DayOfWeek.Friday:

                return true;

​

            case DayOfWeek.Saturday:

            case DayOfWeek.Sunday:

            default:

                return false;

        }

    }

​

    private bool isBeforeEndOfWorkingTime(DateTime dateTime)

    {

        var result = false;

        switch (dateTime.DayOfWeek)

        {

            case DayOfWeek.Monday:

            case DayOfWeek.Tuesday:

            case DayOfWeek.Wednesday:

            case DayOfWeek.Thursday:

                result = dateTime.TimeOfDay < END_WORKING_HOUR;

                break;

            case DayOfWeek.Friday:

                result = dateTime.TimeOfDay < END_WORKING_HOUR_FRIDAY;

                break;

        }

​

        return result;

    }

​

    private DateTime resetToMorningTime(DateTime dateTime)

    {

        var result = dateTime.Date + START_WORKING_HOUR;

        return result;

    }

​

    private DateTime resetToEveningTime(DateTime dateTime)

    {

        var result = dateTime;

        switch (dateTime.DayOfWeek)

        {

            case DayOfWeek.Monday:

            case DayOfWeek.Tuesday:

            case DayOfWeek.Wednesday:

            case DayOfWeek.Thursday:

                result = dateTime.Date + END_WORKING_HOUR;

                break;

            case DayOfWeek.Friday:

                result = dateTime.Date + END_WORKING_HOUR_FRIDAY;

                break;

        }

​

        return result;

    }

}

​

public class TestData

{

    public DateTime InitialDate {get;set;}

    public int DueDays {get;set;}

    public DateTime ExpectedDueDate {get;set;}

    public TestData(DateTime initialDate, int dueDays, DateTime expectedDueDate)

    {

        InitialDate = initialDate;

        DueDays = dueDays;

        ExpectedDueDate = expectedDueDate;

    }

}