using System;
using System.Globalization;


namespace Enum
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = "12/20/2013 17:40";
            var date = DateTime.ParseExact(s, "MM/dd/yyyy HH:mm", CultureInfo.InvariantCulture);
            Console.WriteLine(date.ToString("dd MMM yyyy hh:mm tt", CultureInfo.InvariantCulture));
        }
    }
}