using System;

using System.Globalization;

​

public class Program

{

    public static void Main()

    {

        var dFilterDate1 = DateTime.Now.AddHours(-4);

        var dFilterDate2 = dFilterDate1.AddDays(-4);

        var dFilterDate3 = dFilterDate1.AddDays(-(dFilterDate1.DayOfYear - 1));

        var dFilterDate4 = dFilterDate3.AddYears(-1);

        var dFilterDate5temp1 = ((int)dFilterDate1.DayOfWeek + 1) - 1;

        var dFilterDate5temp2 = (int)dFilterDate4.DayOfWeek + 1;

        var dFilterDate5temp3 = dFilterDate4.AddDays(-dFilterDate5temp2);

        var dFilterDate5temp4 = GetWeekOfYear(dFilterDate1);

        var dFilterDate5temp5 = dFilterDate5temp3.AddDays(7 * (dFilterDate5temp4 - 1));

        var dFilterDate5 = dFilterDate5temp5.AddDays(dFilterDate5temp1);

        Console.WriteLine(dFilterDate1);

        Console.WriteLine(dFilterDate2);

        Console.WriteLine(dFilterDate3);

        Console.WriteLine(dFilterDate4);

        Console.WriteLine(dFilterDate5);

    }

​

    public static int GetWeekOfYear(DateTime dt)

    {

        DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;

        Calendar cal = dfi.Calendar;

        return cal.GetWeekOfYear(dt, dfi.CalendarWeekRule, dfi.FirstDayOfWeek);

    }

}