Write a program in C# Sharp to find the last day of a week against a given date

using System;

class dttimeex53
{
    static void Main()
    {
	int yr,mn,dt;
    Console.Write("\n\n Find the first day of a week against a given date :\n");
	Console.Write("--------------------------------------------------------\n");
	
	Console.Write(" Input the Day : ");
    dt = Convert.ToInt32(Console.ReadLine());	
    Console.Write(" Input the Month : ");
    mn = Convert.ToInt32(Console.ReadLine());	
    Console.Write(" Input the Year : ");
    yr = Convert.ToInt32(Console.ReadLine());		
	DateTime d = new DateTime(yr, mn, dt);
	Console.WriteLine(" The formatted Date is : {0}",d.ToString("dd/MM/yyyy"));
	Console.WriteLine(" The first day of the week for the above date is : {0}\n", FirstDayOfWeek(d).ToString("dd/MM/yyyy"));
    }
   public static DateTime FirstDayOfWeek(DateTime dt)
    {
        var culture = System.Threading.Thread.CurrentThread.CurrentCulture;
        var diff = dt.DayOfWeek - culture.DateTimeFormat.FirstDayOfWeek;
        if(diff < 0) 
            diff += 7;
        return dt.AddDays(-diff).Date;
    }
}