using System;

using System.Globalization;

public class Program

{

    private static CultureInfo enUS = new CultureInfo("en-US");

    private const string STANDARD_DATE_FORMAT = "yyyy-MM-dd HH:mm:ss";

    public static void Main()

    {

        DateTime date1, date2;

        NormalizeServiceRequestDates("2021-06-21 00:00:00", "", out date1, out date2);

        Console.WriteLine(date1.ToString());

        Console.WriteLine(date2.ToString());

        Console.WriteLine("4 hour");

        NormalizeServiceRequestDates("2021-06-21 09:00:00", "", out date1, out date2);

        Console.WriteLine(date1.ToString());

        Console.WriteLine(date2.ToString());        

​

        NormalizeServiceRequestDates("", "2021-06-21 00:00:00", out date1, out date2);

        Console.WriteLine(date1.ToString());

        Console.WriteLine(date2.ToString());

        NormalizeServiceRequestDates("2021-06-21 09:00:00", "2021-06-21 05:00:00", out date1, out date2);

        Console.WriteLine(date1.ToString());

        Console.WriteLine(date2.ToString());

        NormalizeServiceRequestDates("2021-06-21 09:00:00", "2021-06-21 13:00:00", out date1, out date2);

        Console.WriteLine(date1.ToString());

        Console.WriteLine(date2.ToString());

        NormalizeServiceRequestDates("2021-06-21 00:00:00", "2021-06-22 00:00:00", out date1, out date2);

        Console.WriteLine(date1.ToString());

        Console.WriteLine(date2.ToString());

    }

        public static bool NormalizeServiceRequestDates(string strDate1, string strDate2, out DateTime date1, out DateTime date2)

        {

            if (!NormalizeMidnightDateTo8AMTo5PMIfNeeded(strDate1, strDate2, out date1, out date2))

            {

                if (!NormalizeDateTo4HourDurationIfNeeded(strDate1, strDate2, out date1, out date2))

                {

                    return false;

                };

            }

            return true;

        }   

        static bool NormalizeDateTo4HourDurationIfNeeded(string strDate1, string strDate2, out DateTime date1, out DateTime date2)

        {

            bool b1 = DateTime.TryParseExact(strDate1, STANDARD_DATE_FORMAT, enUS, DateTimeStyles.None, out date1);

            bool b2 = DateTime.TryParseExact(strDate2, STANDARD_DATE_FORMAT, enUS, DateTimeStyles.None, out date2);

            if (b1 && (!b2 || String.IsNullOrEmpty(strDate2)))

            {

                date2 = date1;

                date2 = date2.AddHours(4);

                return true;

            }

            return false;

        }   

    static bool NormalizeMidnightDateTo8AMTo5PMIfNeeded(string strDate1, string strDate2, out DateTime date1, out DateTime date2)

    {

        bool dStart = DateTime.TryParseExact(strDate1, STANDARD_DATE_FORMAT, enUS, DateTimeStyles.None, out date1);

        bool dEnd = DateTime.TryParseExact(strDate2, STANDARD_DATE_FORMAT, enUS, DateTimeStyles.None, out date2);

​

        //bool bCheck1 = date1.Hour == 0 && date1.Minute == 0 && date1.Second == 0 && date1.Millisecond == 0 && !dEnd; // If start date of the form "YYYYMMDD 00:00:00" && end date is null/"" -> 8 to 5

​

        bool bIsStartMidnight = date1.Hour == 0 && date1.Minute == 0 && date1.Second == 0 && date1.Millisecond == 0 && dStart;

        bool bIsEndMidnight = date2.Hour == 0 && date2.Minute == 0 && date2.Second == 0 && date2.Millisecond == 0 && dEnd;

​

        if (bIsStartMidnight && !dEnd)

        {

            date1 = date1.ChangeHour(8);

            date2 = date1.ChangeHour(17);

            return true;

        }

​

        if (bIsStartMidnight && bIsEndMidnight)

        {

            date1 = date1.ChangeHour(8);

            date2 = date1.ChangeHour(17);

            return true;

        }       

​

        if (bIsEndMidnight && !dStart)

        {

            date1 = date2.ChangeHour(8);

            date2 = date2.ChangeHour(17);

            return true;

        }

​

        return false;

    }

}

​

public static class Ext

{

    public static DateTime ChangeHour(this DateTime dateTime, int hours = -1)

    {

        return new DateTime(

            dateTime.Year,

            dateTime.Month,

            dateTime.Day,

            hours >= 0 ? hours : dateTime.Hour,

            dateTime.Minute,

            dateTime.Second,

            dateTime.Millisecond,

            dateTime.Kind);

    }

}