ExcelDataReader and Date Field

I am looping through the Excel document saved withing my project (wwwroot) and I cannot get the date to work. I have searched and found a reference to call GetDateTime as opposed to GetValue and also attempted to use ToString() and ToShortDateString(); 

Cannot implicitly convert type 'string' to 'System.DateOnly'

What am I missing?

              using (var stream = System.IO.File.Open(filePath, FileMode.Open, FileAccess.Read))
              {
                  // Auto-detect format, supports:
                  //  - Binary Excel files (2.0-2003 format; *.xls)
                  //  - OpenXml Excel files (2007 format; *.xlsx, *.xlsb)
                  using (var reader = ExcelReaderFactory.CreateReader(stream))
                  {
                      do
                      {
                          while (reader.Read())
                          {
                              BirthModel birth = new BirthModel();
                              birth.LastName = reader.GetValue(1).ToString();
                              birth.FirstName = reader.GetValue(2).ToString();
                              birth.MiddleName = reader.GetValue(3).ToString();
                              birth.BirthDate = reader.GetDateTime(4).ToShortDateString();
                              birth.County = reader.GetValue(5).ToString();
                              birth.Maiden_FirstName = reader.GetValue(6).ToString();
                              birth.Maiden_MiddleName = reader.GetValue(7).ToString();
                              birth.Maiden_LastName = reader.GetValue(8).ToString();
                          }
                      } while (reader.NextResult());