When importing data into Excel, the data that you set will be treated as a Text cell by default. Dates in Excel are equivalent to Delphi TDateTime in that they are a julian date and time is stored as a fraction and share the same numeric equivalents after 3/1/1900 (integer value of 61). This makes life very simple when writing date values to your worksheet since you just need to write the TDateTime value to Excel and if you format the cell as a date, you get the date/time in the worksheet.

procedure TForm1.Button1Click(Sender: TObject);
var
  Excel, Workbook, Worksheet: OleVariant;
begin
  Excel := CreateOLEObject('Excel.Application');
  try
    WorkBook := Excel.WorkBooks.Open('c:\temp\ws.xls');
    WorkSheet := WorkBook.Worksheets.Item['Sheet1'];
    WorkSheet.Range['A1', 'A1'].Value := Now;
  finally
    Excel.Quit;
    Worksheet := Unassigned;
    Workbook := Unassigned;
    Excel := Unassigned;
  end;
end;