using System;

public class Program

{

    static string StartInserted;

    static string EndInserted;

    public static void Main()

    {

        //CalculateDate(null, null); //OK

        //CalculateDate("12-19-2022", null); //OK

        //CalculateDate("11-19-2022", null); //OK

        CalculateDate(null,"12-19-2022"); //OK

        CalculateDate(null,"12-18-2022"); //NOT OK

        CalculateDate(null,"11-19-2022"); //NOT OK

        CalculateDate("11-19-2022", "12-01-2022"); //NOT OK

    }

    public static void CalculateDate(string sDate, string eDate)

    {

        StartInserted = sDate;

        EndInserted = eDate;

        SetConditionalDefaultsNew();

        Console.WriteLine(">= " + StartInserted);

        Console.WriteLine("< " + EndInserted);

        Console.WriteLine("              ");

    }

    public static void SetConditionalDefaultsNew()

    {

        //convert date strings to dates, will return null if no date string or bad date string

        DateTime? ToDateTimeOrNull(string dateString)

        {

            if (string.IsNullOrEmpty(dateString))

            {

                return null;

            }

​

            return DateTime.TryParse(dateString, out DateTime parsedDate) ? parsedDate : (DateTime?) null;

        }

​

        DateTime today = DateTime.Today;

        //convert the input strings to dates

        DateTime? startDate = ToDateTimeOrNull(StartInserted);

        DateTime? endDate = ToDateTimeOrNull(EndInserted);

        //calculate dates when not all dates were provided

        switch (startDate, endDate)

        {

            //no StartInserted and no EndInserted

            case (null, null):

                endDate = today;

                startDate = endDate.Value.AddDays(-3);

                break;

            //StartInserted valid and EndInserted not valid/provided

            case (var s, null) when s.HasValue:

                endDate = startDate.Value.AddDays(3);

                break;

            //StartInserted not valid/provided and EndInserted valid

            case (null, var e) when e.HasValue:

                startDate = endDate.Value.AddDays(-3);

                break;

        }

​

        //end date should not go past tomorrow

        if (endDate >= today)

        {

            endDate = today.AddDays(1);

        }

​

        //TODO: This should be defined in a global place

        string outputDateFormat = "yyyy-MM-ddTHH:mm:ss.fffZ";

        //convert final dates back to string

        StartInserted = startDate.Value.ToString(outputDateFormat);

        EndInserted = endDate.Value.ToString(outputDateFormat);

    }

}

​

public static class StringExtensions

{

    public static string ConvertToDate(this string value, int days = 0, string format = "yyyy-MM-ddTHH:mm:ss.fffZ")

    {

        DateTime result;

        return !DateTime.TryParse(value, out result) ? (string)null : result.AddDays((double)days).ToString(format);

    }

}