Program.cs

using System;

namespace FifthTask
{
    class Program
    {
        private static DateTime _dateOfBirth;
        private static bool _parseSuccessful;

        static void Main()
        {
            ReadDateOfBirth();
            PrintCalculations();
        }

        private static void ReadDateOfBirth()
        {
            do
            {
                try
                {
                    Console.Write("Please enter your birthdate: ");

                    _dateOfBirth = DateTime.Parse(Console.ReadLine());
                    _parseSuccessful = true;
                }
                catch (FormatException)
                {
                    _parseSuccessful = false;
                }
            } while (_parseSuccessful == false);
        }

        private static void PrintCalculations()
        {
            Console.WriteLine("Congratulations! You have been using oxygen on this planet for {0} days so far.",
                              (DateTime.Now.Date - _dateOfBirth.Date).Days);
            Console.WriteLine(
                _dateOfBirth.AddDays(1000) > DateTime.Now
                    ? "You will celebrate your 1000th day alive on {0}."
                    : "You have celebrated your 1000th day alive on {0}.",
                _dateOfBirth.AddDays(1000).ToShortDateString());

            Console.ReadLine();
        }
    }
}