Convert string to date format?

public ActionResult SaveSelectedCustomers(string GetDateTime)
        {
            try
            {
                DateTime getDateTime = DateTime.ParseExact(GetDateTime, "yyyyMMdd HH:mm", CultureInfo.InvariantCulture);
                return Json(true);     
            }
            catch( Exception ex)
            {
                throw ex;
            }
        }

An exception of type 'System.FormatException' occurred in oMail.Web.dll but was not handled in user code

Additional information: String was not recognized as a valid DateTime.