using System;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var settings = new JsonSerializerSettings();

        settings.NullValueHandling = NullValueHandling.Include;

        settings.DefaultValueHandling = DefaultValueHandling.Include;

        settings.Converters.Add(new NullableDateTimeConverter());

        Console.WriteLine("------- not null -------");

        var result1 = JsonConvert.SerializeObject(

            new NullableDateTimeObject { Value = DateTime.UtcNow },

            settings);

        Console.WriteLine(result1);

        Console.WriteLine("------- null -------");

        var result2 = JsonConvert.SerializeObject(

            new NullableDateTimeObject { Value = null },

            settings);

        Console.WriteLine(result2);

        Console.WriteLine("------- explicit converter -------");

        var result3 = JsonConvert.SerializeObject(

            new NullableDateTimeObjectWithExplicitConverter { Value = null },

            settings);

        Console.WriteLine(result3);

        Console.WriteLine("------- but it reads null -------");

        var resultRead = JsonConvert.DeserializeObject<NullableDateTimeObject>(

            "{\"Value\":null}",

            settings);

        Console.WriteLine(resultRead.Value);

    }

}

​

public class NullableDateTimeObject

{

    public DateTime? Value { get; set; }

}

​

public class NullableDateTimeObjectWithExplicitConverter

{

    [JsonConverter(typeof(NullableDateTimeConverter))]

    [JsonProperty(IsReference = true, DefaultValueHandling = DefaultValueHandling.Populate, NullValueHandling = NullValueHandling.Include)]

    public DateTime? Value { get; set; }

}

​

public class NullableDateTimeConverter : JsonConverter

{

    public override bool CanConvert(Type objectType)

    {

        Console.WriteLine("object type: " + objectType.Name);

        return objectType == typeof(DateTime?);

    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)

    {

        if (reader.Value == null)

        {

            return DateTime.Now;

        }

        else

        {

            return (DateTime?)reader.Value;

        }

    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)

    {

        if (value == null)

        {

            serializer.Serialize(writer, DateTime.UtcNow.ToString("o"));

        }

        else

        {

            serializer.Serialize(writer, ((DateTime?)value).Value.ToString("o"));

        }

    }

}