OnTime Fuction

I am trying to get the following macro to run every half hour from 9am until 4pm.

I did try entering the following statement within the macro but I get the error message "compile error - arguement not optional"

Application.OnTime
TimeValue ("16:31:00"), "Mail_Selection_Range_Outlook_Body"

So here is my full macro. Thanks in advance for the help. (Using excel 2003)

Sub Mail_Selection_Range_Outlook_Body()

Dim rng As Range
Dim OutApp As Object
Dim OutMail As Object
Set rng = Nothing
On Error Resume Next
'Only the visible cells in the selection
Set rng = Sheets("Sheet1").Range("A1:C41").SpecialCells(xlCellTypeVisible)
'You can also use a range if you want
'Set rng = Sheets("YourSheet").Range("D4:D12").SpecialCells(xlCellTypeVisible)
On Error GoTo 0
If rng Is Nothing Then
MsgBox "The selection is not a range or the sheet is protected" & _
vbNewLine & "please correct and try again.", vbOKOnly
Exit Sub