using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        var format = "yyyy/M/ddd/dd";

        var provider = new CultureInfo("en-GB"); // Have tried with several other cultures including Invariant

        var datetime = new DateTime(4444, 3, 2, 1, 5, 6, 777);

        var datetimeAsString = datetime.ToString(format, provider);

​

        // Here datetimeAsString == "4444/3/Wed/02"

        var x = DateTime.ParseExact(datetimeAsString, format, provider);

        Console.WriteLine(x);

    }

}