string str = "1st February 2013";
DateTime dtObject;
string replacedStr =  str.Substring(0,4)
                         .Replace("nd","")
                         .Replace("th","")
                         .Replace("rd","")
                         .Replace("st","")
                         + str.Substring(4);


if (DateTime.TryParseExact(replacedStr,
                            "dd MMMMM yyyy",
                            CultureInfo.InstalledUICulture,
                            DateTimeStyles.None,
                            out dtObject))
{
 //valid date
}

string[] formats= {"M/d/yyyy h:mm:ss tt", "M/d/yyyy h:mm tt",
                   "MM/dd/yyyy hh:mm:ss", "M/d/yyyy h:mm:ss",
                   "M/d/yyyy hh:mm tt", "M/d/yyyy hh tt",
                   "M/d/yyyy h:mm", "M/d/yyyy h:mm",
                   "MM/dd/yyyy hh:mm", "M/dd/yyyy hh:mm"};
string[] dateStrings = {"5/1/2009 6:32 PM", "05/01/2009 6:32:05 PM",
                        "5/1/2009 6:32:00", "05/01/2009 06:32",
                        "05/01/2009 06:32:00 PM", "05/01/2009 06:32:00"};
DateTime dateValue;

foreach (string dateString in dateStrings)
{
   if (DateTime.TryParseExact(dateString, formats,
                              new CultureInfo("en-US"),
                              DateTimeStyles.None,
                              out dateValue))
      Console.WriteLine("Converted '{0}' to {1}.", dateString, dateValue);
   else
      Console.WriteLine("Unable to convert '{0}' to a date.", dateString);