using System;

using System.IO;

using Newtonsoft.Json;

​

public class Program

{

    public static void Main()

    {

        var dateWithoutMilliseconds = new DateTime(3000,1,1,0,0,0);

        var dateWithMilliseconds = new DateTime(3000,1,1,0,0,0,999);

        var json1 = JsonConvert.SerializeObject(dateWithoutMilliseconds);

        var json2 = JsonConvert.SerializeObject(dateWithMilliseconds);

        Console.WriteLine("Newtonsoft: " + json1);

        Console.WriteLine("Newtonsoft: " +json2);

        var json3 = System.Text.Json.JsonSerializer.Serialize<DateTime>(dateWithoutMilliseconds);

        var json4 = System.Text.Json.JsonSerializer.Serialize<DateTime>(dateWithMilliseconds);

        Console.WriteLine("STJ       : " + json3);

        Console.WriteLine("STJ       : " + json4);

    }

}