using System;

​

public class Program

{

    public static void Main()

    {

        DateTime endDate = DateTime.Parse("2019-10-31");

        DateTime start1M = endDate.AddMonths(-1);

        DateTime start3M = endDate.AddMonths(-3);

        DateTime start6M = endDate.AddMonths(-6);

        Console.WriteLine("End Date: " + endDate.ToString());

        Console.WriteLine("Start Date 1M ago: " + start1M.ToString());

        Console.WriteLine("Start Date 3M ago: " + start3M.ToString());

        Console.WriteLine("Start Date 6M ago: " + start6M.ToString());

    }

}