using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var atividades = new List<ActivityGuided>()

        {

            new ActivityGuided{ Id = 1, StartDate = DateTime.Now},

            new ActivityGuided{ Id = 2, StartDate = null}

        };

        foreach (var item in atividades.OrderBy(x => x.StartDate))

        {

            Console.WriteLine(item.Id + " - " + item.StartDate);

        }       

    }

    public class ActivityGuided

    {

        public int Id { get; set; }

        public DateTime? StartDate { get; set; }

    }

}