The SQL Server version

CREATE FUNCTION [dbo].[fISODate] (@InputDate DATETIME) returns varchar(8)
AS
begin
declare @Return varchar(8)
select @return =
	substring(convert(varchar, @InputDate, 101),7,4) +
	substring(convert(varchar, @InputDate, 101),1,2) +
	substring(convert(varchar, @InputDate, 101),4,2)
return @return
end

The VBA version

Public Function fISODate(InputDate As Variant) As String
 If IsNull(InputDate) Then
  fISODate = "00000000"
 Else
  fISODate = Format(InputDate, "yyyymmdd")
 End If
End Function