Program.cs

using System;
using System.Text.RegularExpressions;
using System.Threading;

namespace Stopwatch
{
    class Program
    {
        static void Main(string[] args)
        {
            Menu();
        }

        static void Menu()
        {
            Console.Clear();

            Console.WriteLine("Welcome to the stopwatch! Select how long you want to count below:");
            Console.WriteLine();

            Console.WriteLine("S => Seconds (ex: 10s = 10 Seconds)");
            Console.WriteLine("M => Minutes (ex: 1m = 1 Minute)");
            Console.WriteLine("E => Exit");

            Console.WriteLine();
            Console.Write("Enter your choice: ");

            string value = Console.ReadLine().ToLower();

            if (value == "e")
            {
                System.Environment.Exit(0);
            }
            else if (!Regex.IsMatch(value, @"^\d+[sm]$"))
            {
                Console.WriteLine("Invalid Format!");
                Console.ReadKey();
                Menu();
                return;
            }

            char type = char.Parse(value.Substring(value.Length - 1, 1));
            int time = int.Parse(value.Substring(0, value.Length - 1));

            int multiplier = type == 'm' ? 60 : 1;

            PreStart(time * multiplier);
        }

        static void PreStart(int time)
        {
            Console.Clear();
            Console.WriteLine("Ready...");
            Thread.Sleep(1000);

            Console.WriteLine("Set...");
            Thread.Sleep(1000);

            Console.WriteLine("Go...");
            Thread.Sleep(2500);

            Start(time);
        }
        static void Start(int time)
        {
            int currentTime = 0;

            while (currentTime != time)
            {
                Console.Clear();

                currentTime++;
                Console.WriteLine(currentTime);

                Thread.Sleep(1000);
            }

            Console.Clear();
            Console.Write("Stopwatch Finish!");
            Thread.Sleep(2500);

            Menu();
        }
    }
}