Você pode usar DateTime.TryParseExact para
datetime
personalizado formato como: string strDateStarted = "19-MAR-13 08.13.27 AM";
DateTime datDateStarted;
DateTime.TryParseExact(strDateStarted, new string[] { "dd-MMM-yy hh.mm.ss tt" }, System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.None, out datDateStarted);
Console.WriteLine(datDateStarted);