using System;

public class Program

{

    public static void Main()

    {

        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)

    {

        return dateTime?.ToString("MM/dd/yyyy") ?? "Unknown date";

    }

}

​