using System;

using System.Collections;

using System.Collections.Generic;

using System.Globalization;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

​

public class Program

{

    public static void Main()

    {

        var serialized = JsonConvert.SerializeObject(new MyClass{dt = new TimeOnly(11, 1, 1)});

        Console.WriteLine(serialized);

        var de = JsonConvert.DeserializeObject<MyClass>(serialized);

        Console.WriteLine(de.dt);

    }

}

​

public class TimeOnlyJsonConverter : JsonConverter<TimeOnly>

{

    private const string Format = "HH:mm:ss.FFFFFFF";

    public override TimeOnly ReadJson(JsonReader reader, Type objectType, TimeOnly existingValue, bool hasExistingValue, JsonSerializer serializer) => TimeOnly.ParseExact((string)reader.Value, Format, CultureInfo.InvariantCulture);

    public override void WriteJson(JsonWriter writer, TimeOnly value, JsonSerializer serializer) => writer.WriteValue(value.ToString(Format, CultureInfo.InvariantCulture));

}

​

class MyClass

{

    [Newtonsoft.Json.JsonConverter(typeof(TimeOnlyJsonConverter))]

    public TimeOnly dt { get; set; }

}