using System;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        string json = @"[{""Name"":""Joe Schmoe""}]";

        List<Person> people = JsonConvert.DeserializeObject<List<Person>>(json);

        people.Add(new Person { Name = "Jane Doe", Birthday = new DateTime(1988, 10, 6) });

        json = JsonConvert.SerializeObject(people, Formatting.Indented);

        Console.WriteLine(json);

    }

}

​

public class Person  // Version 2

{

    public string Name { get; set; }

    public DateTime? Birthday { get; set; }

}