using System;

public class Program

{

    public static void Main()

    {

        DateTime veryKnownDate = DateTime.Now;

        Console.WriteLine("Date is very provided: " + veryKnownDate.ToDateString());

        DateTime? knownDate = DateTime.Now;

        DateTime? unknownDate = null;

        Console.WriteLine("Date is provided:      " + knownDate.ToDateString());

        Console.WriteLine("Date is not provided:  " + unknownDate.ToDateString());

    }

}

​

internal static class Helpers

{

    public static string ToDateString(this DateTime? dateTime)

    {

        if (dateTime == null)

        {

            return "Unknown date";

        }

        return dateTime.Value.ToDateString();

    }

    public static string ToDateString(this DateTime dateTime) => dateTime.ToString("MM/dd/yyyy");

}

​