DateTime Issue(string is not a valid datetime)

You coudlt not convert .MM/dd/yyyy format only convert Datetime data type other wise you coudlt not convert,Date Time data type convert only MM/dd/yyyy format only chage that after only you will convert This mistake do all the developer first time

public const string DEFAULT_DATE_FORMAT = "dd/MM/yyyy";
public static string ConvertDtFormat(string dateTimeString)
           {
               dateTimeString = dateTimeString.Trim();
               while (dateTimeString.Contains("  "))
               {
                   dateTimeString = dateTimeString.Replace("  ", " ");
               }

               if (dateTimeString == null || dateTimeString.Length == 0)
               {
                   return string.Empty;
               }
               else
               {
                   DateTime convertedDateTime = new DateTime();
                   string userDateFormat =DEFAULT_DATE_FORMAT; 

                   try
                   {

                       if (userDateFormat.Trim().Contains("dd/MM/yyyy"))
                           convertedDateTime = DateTime.ParseExact(dateTimeString, format_dmy, CultureInfo.InvariantCulture,
                                                                   DateTimeStyles.AllowLeadingWhite | DateTimeStyles.AllowTrailingWhite);
                       else if (userDateFormat.Trim().Contains("MM/dd/yyyy"))
                           convertedDateTime = DateTime.ParseExact(dateTimeString, format_mdy, CultureInfo.InvariantCulture,
                                                                   DateTimeStyles.AllowLeadingWhite | DateTimeStyles.AllowTrailingWhite);

                       return convertedDateTime.ToString("MMM dd, yyyy hh:mm tt");
                   }
                   catch
                   {
                       return "Invalid DateTime";
                   }
               }
           }