convert date time format !! please help

using System;

public class Program
{
    public static void Main()
    {
          // change 2/22/2014 5:08:46 PM to 2014-02-22.
          string dateString = "2/22/2014 5:08:46 PM";

          Console.WriteLine("The original string is {0}", dateString);

          DateTime dt;

          if (DateTime.TryParse(dateString, out dt))
          {
               string format = "yyyy-MM-dd";
               Console.WriteLine(dt.ToString(format));
          }
          else
          {
              Console.WriteLine("Not a valid datetime");
          }
    }
}