using System.Globalization;

using System;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Enter the date (if you want to enter a date from more than 99 years ago, you can enter the date in the dd/MM/yyyy format):");

        string dateString = Console.ReadLine();

        DateTime date;

        CultureInfo ci = (CultureInfo)CultureInfo.InvariantCulture.Clone();

        ci.Calendar.TwoDigitYearMax = DateTime.Now.Year;

        if (!DateTime.TryParseExact(dateString, new string[]

        {

        "dd/MM/yyyy", "dd/MM/yy"

        }

​

        , ci, DateTimeStyles.None, out date))

        {

            throw new InvalidOperationException("Not a valid date");

        }

​

        Console.WriteLine(date.ToString("dd/MM/yyyy", CultureInfo.InvariantCulture));

    }

}