using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        string ds = "Posted on Thursday, May 1st, 2014 at 10:07 AM";

​

        string[] formats = new string[] {

            "MMMM d'st, 'yyyy' at 'hh:mm tt",   // for parsing like "1st"

            "MMMM d'nd, 'yyyy' at 'hh:mm tt",   // for parsing like "2nd"

            "MMMM d'rd, 'yyyy' at 'hh:mm tt",   // for parsing like "3rd"

        };

​

        // Get rid of the "Posted on Thursday"

        ds = ds.Substring(ds.IndexOf(", ") + 2);

        DateTime date = DateTime.ParseExact(ds, formats, null, DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal);

​

        // Once you date a DateTime, you can format it any way you want

        Console.WriteLine(date.ToString("o"));

    }

}