Custom DateTime serialization with Json.Net

I think you are overcomplicating things. Your converter should only worry about converting date values, not objects that may contain date values. The serializer will call your converter naturally whenever a date is encountered inside any other object.

Here is a simple converter that will do what you want:

public class MinDateToNullConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        // This converter handles date values directly
        return (objectType == typeof(DateTime));
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        // The CanConvert method guarantees the value will be a DateTime
        DateTime date = (DateTime)value;
        if (date == DateTime.MinValue)
        {
            writer.WriteNull();
        }
        else
        {
            writer.WriteValue(date);
        }
    }

    public override bool CanRead
    {
        get { return false; }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

Here is a demo showing that the converter works on dates throughout a nested hierarchy of objects:

class Program
{
    static void Main(string[] args)
    {
        Example example = new Example
        {
            Date1 = new DateTime(2014, 2, 2),
            Date2 = DateTime.MinValue,
            Inner = new Inner
            {
                DateA = DateTime.MinValue,
                DateB = new DateTime(1954, 1, 26)
            },
            MoreDates = new List<DateTime>
            {
                new DateTime(1971, 11, 15),
                DateTime.MinValue
            }
        };

        // Set up the serializer to use our date converter
        JsonSerializerSettings settings = new JsonSerializerSettings();
        settings.Converters.Add(new MinDateToNullConverter());
        settings.Formatting = Formatting.Indented;

        string json = JsonConvert.SerializeObject(example, settings);
        Console.WriteLine(json);
    }
}

class Example
{
    public DateTime Date1 { get; set; }
    public DateTime Date2 { get; set; }
    public Inner Inner { get; set; }
    public List<DateTime> MoreDates { get; set; }
}

class Inner
{
    public DateTime DateA { get; set; }
    public DateTime DateB { get; set; }
}

Output:

{
  "Date1": "2014-02-02T00:00:00",
  "Date2": null,
  "Inner": {
    "DateA": null,
    "DateB": "1954-01-26T00:00:00"
  },
  "MoreDates": [
    "1971-11-15T00:00:00",
    null
  ]
}

When converter implements CanConvert (and yours does), you can add it to serializer settings:

JsonConvert.SerializeObject(foo, new JsonSerializerSettings {
    Converters = {
        new DateTimeConverter()
    }
});

This way, it will be applied to all objects it supports.

Found the answer to my question. Actually I had 2 converters in JsonSerializer settings. I noticed that when I removed the converter on object A which contains DateTime MinValue property, My serializer works fine. This led me understand that the converter of DateTime to null is not working under other converter. So what I did is converting manually inside object A converter, not exactly clean solution, and I would prefer that the DateTimeToNull Converter will also work inside A converter. Here is what I have done inside A converter:

    public class AConverter : JsonConverter
{

    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(A));
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        JToken jToken = JToken.FromObject(value);
        JObject jObject = (JObject)jToken;
        A aInfo = (A) value;
        string statusString = aInfo.Status == null ? string.Empty : SomeUtil.GetStateString((State)aInfo.Status.State, aInfo.Status.Downloading);
        jObject.AddFirst(new JProperty("MachineState", statusString));

        if (aInfo.UploadTime == DateTime.MinValue) {
            jObject.Remove("UploadTime");
            jObject.Add(new JProperty("UploadTime", null));
        }
        jObject.WriteTo(writer);
    }

    public override bool CanRead
    {
        get
        {
            return false;
        }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException("It's OK!");
    }


}