using System;

using System.Net;

public class Program

{

    public static void Main()

    {

        var request = (HttpWebRequest) WebRequest.Create("https://pt.stackoverflow.com/questions/236876/como-criar-um-header-do-tipo-date-mas-na-forma-de-iso-8601");

        request.Date = DateTime.UtcNow.AddHours(-1);

        Console.WriteLine(request.Date);

        using(var response = (HttpWebResponse)request.GetResponse())

        {

            Console.WriteLine(response.StatusCode);

        }

    }

}