.NET Fiddle

CSharp · Console

/Program.cs

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);
	}
}